Created the bar in InDesign, with 100% black fill, 7% tint in colour. Exported prepress PDF page for print house. However, printed copies show with inconsistency shading on bar throughout whole book. Not sure why, doesn’t showed on screen. Appreciate anyone with similar experience can provide some expert advice. Thanks. 
Were there no gradients involved? Just a 7% tint of black ink?
Was the book printed on an offset or digital press? Did the printer supply you with a proof that looked fine? If so, it was a printing press problem having to do with uneven ink distribution. You should request that they print it over for free since the proof doesn’t match the final printed book.
Is this a self-published book printed by one of the online book publishers, such as KDP, Lulu, BookBaby, or one of the others (I don’t mean to single one of them out)? If so, I’m not especially surprised. The quality standards of these print-on-demand printers are not good.
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Anything near or close to 5% is pushing the limits of printing any tint. Plus there is Delta E to be taken into account (in simple terms, the allowable drift on a print machine.) Also if you used a K black or a CMYK build of some kind (with the latter introducing more variable results.)
Like B said,
Talk you your printer.
Point to your proof.
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@PrintDriver makes some good points. A <7% screen tint on a bar that runs across the width of all the pages on the inside of a book is a design element that would be especially prone to the kind of printing problem you mentioned. A conscientious, quality-oriented printer could have done it, but if it had been me, I would have avoided taking the chance.
Even so, if they sent a proof showing a consistent tint, and if you signed off on it, the proof is their commitment that the printed job will look like the proof. However, if you were working with an online print-on-demand gang printer, I have no idea what kind of contractual gotchas you agreed to in the fine print.
For what it’s worth, this is the kind of problem that wouldn’t have shown up on a digital proof since it’s a press problem. However, if they weren’t able to confidently print what you sent them, they should have told you beforehand.
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Thank you all for your valuable opinions.
It was solid black with 7% tint, item created in InDesign, no printed proof, but only digital proof were supplied. The inconsistency of the solid black 7% tint were shown on the digital proof (later they found inconsistency shown on the digital after this print issues arise.)
I am not sure it was printed as offset or digital. Will that make the differences?
I am concerned that the 7% tint on any colours will cause this issues, regardless digital or offset print. Even with the traditional films output. I might be wrong??
It might make a difference in explaining what happened and the terminology I use, but not in whether or not it was their fault.
I’m skeptical of that explanation. If you created a box in InDesign and specified that it be filled with 7% K, I’m having difficulty imagining what you could have done to create the inconsistency that they claim to have subsequently found in the digital proof. There’s really not a whole lot that could go wrong unless you did something really odd, but I’m assuming that’s not the case.
If you’re seeing an inconsistency in the proof, I’m at a loss to explain it. A shape with a fill is just about the simplest bit of code possible in InDesign. Their output device’s RIP would have no problem with it. I’d really like to see that digital proof for myself.
Unless there’s a better explanation of something not immediately obvious from what you’ve written, to me, it sounds like they just had a problem holding a consistent 7% dot on the press and didn’t notice or didn’t care.
Film would even be more problematic since there’s an extra analog step in the process where tiny dots can burn out of the film.
A good offset printer can print a 1–2 percent dot, but I would never send a printer anything with a tint that light. I’m unsure what you could get from a good digital printer, but I doubt it’s any better. I’d never send anything to an online gang printer (or an online book printer) that specified a tint less than 7 percent. I routinely send files to good, quality-oriented printers with objects composed of process color mixes where one of the inks is four or five percent, but I’d be a little wary of printing a large area with a light tint for exactly the problem you’ve seen — visibly inconsistent ink coverage across the width of the paper. As I mentioned, though, I wouldn’t mind seeing that digital proof (and a close-up image showing the printed inconsistency).
I haven’t seen the physical copy myself, the information was from my co-worker. Regarding the digital proof the printer sent, again, I haven’t seen it and when they tried to re-viewed it, it was expired. Your comments validate my doubt on inconsistency in the digital proof, when they said they can see it on screen.
I think that it is to avoid any <7% tinted screen in the future.
I agreed, I previously worked in a film output before this CTP came on-board.
Thank you all for your comments/feedbacks and input. Very appreciated! 