T35 Touch Screen, sometimes does not fire

I have a program that defines 7 touchdown buttons on a T-35 Display. On occasion one or two of the touchdown buttons will not fire, or requires I touch the offending button multiple times (like 10 or more). I was wondering if the temperature, humidity, etc. could have an effect? Also, is there a technical advantage of touchdown vs touchup?
Thanks, Bob

Do you get this if using a stylus?

@ Gus -
I dont have a stylus. If one came with the display I may have misplaced it. Simply touching the screen with my finger.

Strange, right now it is working fine. No rhymn or reason that I can think of. Difficult to recreate the problem with any consistency.