Screen is touch sensitive only when held

Hi Guys,

Thanks for your time reading this from a long time lurker,

Something that I've noticed recently whilst testing a bunch of 5c repairs. Its the weirdest thing i personally haven't noticed before but want to get an opinion from you interweb experts.

Picture this, A neat row of 5c's on my worktop all ready to be run thru the iOS setup process prior to testing. Some work flawlessly, reacting to the merest brush of my little pinky, others are not as nice. They insist on being held for the screen to even think about reacting.

Has this been heard of before? It is a known thing that I totally missed?

Could it be an issue with the repair process, maybe with the parts used or could it have been a thing since the 5c release and I've only just noticed. Something to do with the plastic chassis and the capacitive screen perhaps.

Thanks for your future input on this,