hi, me and my buddy were out shooting today and i had a problem that i have never had before. i shot my 870 and when went to pump it to load another shell, it did not grab the fired shell and tried to load a shell with the empty one still in the chamber. it did this twice and my buddy thought it was cause the gun was dirty but this was the first time i used it since i cleaned it last. i clean it pretty regularly. anybody know what may be causing this.