I don't remember for sure, but I think that what solved the problem for me was replacing the digitizer with a new one from a seller that I trust. I am not sure why my digitizer worked beautifully at first and then failed. Possibilities include small rips, kinks, or slightly bad/dirty connection points at the end of the cable. Or slightly incomplete insertion into a connector. This last is worth checking: open the latches on both connectors, carefully push the cable in with tweezers as far as it can go, and close the latches.
In my case, possibly there was damage to the cable that was only revealed when the assembly was tightly closed.
I think that I did insulate the edges of the digitizer with Kapton tape, as has been suggested elsewhere, and that did not help in my case.
Another possibility if it is not the digitizer, is that one of the connectors is faulty. If you have a microscope or even magnifying glass, inspect the solder joints between each pin and pad, making sure that they look good and have the classic "ski slope" form of a good joint.
If you have a multimeter, you can also test continuity between each pad and the corresponding pin. Look not only for continuity, but also the absence of any bridges: each pad should be continuous with its pin and its pin only.
If you have ZXW and know which pins should be ground, you can test that as well. If you are fortunate enough to have a chart of what the diode readings should be for each pin (or a "known good" with which to compare) you can also test the diode reading of each pin.
To my knowledge and in my experience, dead spots are unlikely to be caused by damage to the backlight circuit. Backlight damage is usually reflected by either a half dim screen or one that is completely blank.
In sum, I think you are left with the digitizer itself (damage to the cable, incomplete insertion into a connector, or lack of insulation around the edges) or with one or both of the digitizer connectors on the motherboard.