Back glass broke soon after replacement

After doing my replacement I had a feeling that the glass back panel I had ordered wasnt quite sitting right on one of the two devices I made repairs to. After cleaning the metal bezel of the device several times over, dry-fitting the part lined up correctly but actually applying the part, it wouldnt go down all the way past flush to the bezel. So while letting the adhesive that came with my order set, I put the phone under a ~10lb textbook on a table. After a month of use, the back glass separated on one corner, so I softened the adhesive with the iopener and put it back under the textbook, and it looked fine again. Later that week, I set the phone down on a wood table and heard a crunch, the back had cracked spectacularly. I was wondering if its possible that I had not used enough force to help set the adhesive, or that maybe this specific piece of glass wasn't up to par with the standard. I'm quite familiar with phone repair, so I'm not sure what happened. The factory back glass that I was able to reuse on the second device hasn't had an issue yet, and the adhesive set to the correct depth. Any insight would be appreciated, thank you!