---------BT99 Problem---------

I just bought a older bt-99 (RT) about 4-5 weeks ago, the gun has been working great until this last weekend. I was just practice shooting at the club and when I went to open up the gun to eject the shell, it did not want to open as easy as always? there was a couple times I thought it was not going to open. I had to push on the lever pretty hard to get it to open up, I took the gun home and gave it a good cleaning and noticed that with out the barrel on, the lever moves freely like before, but when I install the barrel, its hard to release it again. would anyone here with all of you knowledge know what has possibly happened.

Thanks in advance
Roger