It's gonna be tough to do, but while the slide is all the way in check to make sure there is nothing on the roof of the slide to prevent it from going all the way out. This happened to my cousin. A tree branch somehow made it past the top slide seal and became lodged between the camper frame and the sealing edge of the slide itself. The stick was about an inch in diameter and like 9 inches long. As the slide came in it dragged the branch perpandicular to the frame prevening it from going out all the way. This may not be tour problem, but it might be though. Do you have a slide awning? If not, it's entirely possible.
"Walk softly...and carry a lot of tools!
2016 Ford F250 Super Duty Crew Cab, 4x4
2021 Palomino Puma 27RLSS