Make yourself a bead of polymer clay, or playdoh, or whatever on a plain mandrel to see what size glass you REALLY need to use the press--then round it back up & set it on your table to use as a visual guide for "how much glass" to use for that press--if your mandrel isn't fitting in the press, either you're using too large a mandrel (possible) or you're pressing too much glass (likely) or both
~luna
been there, done that