it may not be obvious but are the travel bars installed?
if not, our slide stuck quite often when we first had it. found out because we didn't have a slide topper installed, residue, sap and grime got on the seals and became like a glue. i couldn't even push it apart. i had to wedge something between the seals and the side of the trailer, to break loose the adhesion between them.
we quickly had a topper installed and have never had this issue since.
one reason is that we camp a lot in forested areas with lots of tree residue.