I had this problem too, dampen you finger and it should work, mine mine does and I have a glass screen protector fitted. It will probably be fixed with future updates.
The same problem with my phone. Sometimes works but most of the cases it's not (especially when first attempt fail). In parallel, my old phones (Realme 7Pro and Nokia 7.1) works without any problems and much faster. I thought new Zenfone is a bit high-end phone....but looks it's not. I still believe it will be improved with new software but for now it's not worth to buy because of that problem.
Maybe issues with scanner are related to backlight? Too bright maybe etc. and that produces image with too little contrast (overexposure?). I noticed that FP scanner on oneplus 7 (runs like capacitive in blink of an eye) has green backlight instead of white.
Backlight of FP is so bright and strong on ZF8 that it lights through finger.
I don't mind if unlocking takes second - but it's random success in unlocking is irritating.
My latest observation concernig FP scanner (might be wrong). It works more reliable and faster when I press quickly and hold finger. If I try to put finger slower it takes more time to unlock. Maybe there is some problem connected with touch digitizer? It senses finger a little too early and try to scan fingerprint when it is not placed entirely on scanner?
There are some good features of that FP scanner. That you don't have to wake screen to make it work. Just press if you remember where it is located even without AOD.
after 2 days of usage, FP scanner started to be surprisingly accurate and faster (not the fastest though) - using harder to read finger.
Interesting... is there some machine learning?
Again it is hit or miss. Once fast, once slow, once not recognizing. Are you doing something with that? Is it software or hardware problem?
I still think it is a problem with too strong backlight.
FP is significantly improved on my side.
I thought the same but a few hours later again it started to be hit or miss
After returning from repair, I tested the fingerprint sensor without the protective glass - it worked very well. Today I glued a hybrid glass with a thickness of 0.16 mm and I can see a decrease in performance, but it still works quite effectively. before the repair, I had a very good glass but thickness of 0.3mm, unfortunately with such a protector, the fingerprint sensor is useless.
Huawei P30 with 0.3mm glass (ten same model) sensors still work decently.
I have a hybrid glass 0.3mm and after 3 weeks sensor started to be much more reliable - I don't know why , maybe repeated presses of finger somehow better glued glass to then screen?
But I confirm that before application of protective glass it was ok.
For me great feature is that sensor is active on screen off without AOD or lock screen.