Is there a fired round in there when it's hard to open, because that's a different kind of problem.
Assuming this problem happens when there's nothing in the chamber, couple simple things to try first:
First step is always to clean everything. Put it back together with no oil or lube except for maybe a light wipe-down with a slightly oily cloth. Completely disassemble the bolt (it's easy to do) and clean everything well. People put gobs of oil on everything and all that does is gum things up with carbon and dirt and grime. After a while this will get waxy and make things stiffen up. If you need a guide for bolt maintenance, youtube "Anschutz 64 bolt disassembly."
Check your action screws to make sure they're not interfering with the bolt - I really doubt this is the problem, but worth a look.
The next thing to look at is the trigger itself. There is an adjustment for sear engagement. There are multiple triggers for these 64s, and they're all a little different, but too much sear engagement could be the culprit here. If you're goig to adjust this yourself, be careful - you can make your rifle unsafe pretty easily, and even the tiniest adjustment ca make a big difference.
Good luck - hope it's something simple.