Hi Paul. Sometimes, when you purchase plastic sheet, there is one side (the shiny side) that has a very fine film of clear plastic attached to it, designed to protect the surface from scratches. This stuff can be very hard to see and needs a bit of patience to find a corner to get a hold of, in order to remove it in one go. Sounds daft but it's just a possibility.
With regard to masking tape, when you have finished applying the last coat of paint, it is advisable to remove the tape after about 20 - 30 minutes i.e. while the paint is still "going off". Leaving it for 24 hrs would risk pulling the dried paint off (been there, done that). Also, when removing the masking tape, pull it back against itself. This also helps prevent unwanted paint removal.
Hope this helps in some way.
Regards,
Ray.