All adhesives have a weak point! Thats the surface they are trying to adhere. I’m sure thats what you are facing a dirty surface!
What you use to prep the surface could be leaving an oily film or other material thats interfering with the adhesion. I use a three step process to make sure the surfaces are clean.
The first is using distilled water, Yep water! Using a shop wipe and/or cotton swab I go over the area with a cloth or swab slightly damp to clean off the sugars and salts that are on the surfaces going over it a few times gets this done. I’ve personally stopped working on the AppleWatches but I often found this was needed. If the parts still have a lot of adhesive left I sometimes do this second.
The second is to remove any traces of the old adhesive as well as any skin oils. Here I use Goof Off. This will dissolve the old adhesive but it does leave an oily residue! So now we need to clean it off either allowing it to evaporate over a few days.
Or, use reagent grade isopropyl alcohol 90% or higher. The stuff you get over the counter is not clean! This is where I find many people get into trouble! The residues within it leaves a film which interferes with the adhesion. This also will remove any grease or wax as well. I often need to go over the areas a few times to make sure I get it all off.
The last issue is making sure not to touch the surfaces with bare skin! use tweezers and a small metal spatula in the iFixit tool kit. Make sure you wipe them down before you use them.