Went to shoot my 'new to me' c9 for the first time and after 1 mag the slide locked open. Drop the mag, insert a new one and now it won't release. Tried taking the mag out putting in an empty one, nothing. It is locked back and will not release. I did take the slide off recently and polished the feed ramps. Did I put something back wrong? Easy fix or send it back to HP? BTW, the first 8 rounds were great. Fed smooth and very acurate. Thanks in advance.