PBS Lifting HEK Cells: UPDATE

Hi all,

I posted about a week ago talking about how PBS seems to knock my HEK cells loose even though I add it dropwise to the side of the well.

Well, today I got a plate of fully confluent HEK cells from another lab on a 10cm dish. This 10cm dish was different than the one I was culturing in, so it made me think - am I just using a sterile plate that isn't necessarily tissue-culture treated? (The box the plates came in was thrown out and the sleeves just have a "sterile" sticker on them. I think this may be the case as this plate I used the HEK's were STUCK even after trypsinization for 2 minutes, I had to hit the plate a few times to get them loose.

It's either the plate or the PBS I borrowed from a separate lab, but... I'm leaning towards the plate.