I get the same problem sometimes, though I have astral projected right out many many times in the past.
It often makes me wonder why that happens sometimes I've tried to analyze what the reason might be in my case. I've found if I'm in a bit of an "unsure" mood, or lacking confidence, or a little bit upset so I can feel my aura "tighten up" (hard to describe but it feels like I'm all tight in against myself, and not radiating out...) -then that problem is more likely to come up.
Also trying too hard can cause it.
Sometimes a subconscious fear or apprehension about what's on the other side can cause it.
Certainly feeling more confident and more relaxed will help. So that's where a little prayer for guidance and support from Higher Powers (Angelic or Divine) will help.