That's a pretty common issue when using the stock adhesive that is pre-applied to the digitizer.
Here are a few general Best Practices when installing an iPad digitizer.
- You have to properly clean the previous adhesive from the housing. You can use IPA or Acetone to soften up the adhesive, and then scrape away most of it with a flat tool (be careful of the inner flexes and components). I follow that with a solvent imbued wipe to clean the rest off. I like to finish with an IPA wipe and some techs like to use an adhesive Primer.
- Remove the pre-installed adhesive on the digitizer. You can also remove the adhesive residue on the digitizer but you have to be careful of not getting the inside surface of the digitizer smudged with solvents and fingerprints. Hopefully you kept the thin protective sheet on the digitizer up to this point.
- Make sure the flexes are properly sitting within the device, as even the tiniest amount of pressure will eventually cause the adhesive to let go of the digitizer.
- Finally, these adhesive are pressure-activated so you need to apply pressure. You can stack some books on the iPad to get an even pressure distribution.
The actual adhesive matters also. I find that the pre-installed adhesive is generally low quality and doesn't stick as well as a high quality adhesive, such as the Tesa 61395 Tape you can find here on iFixit or pre-cut 3M tape from Bowler-Tech, which is what I use.