Custom failures!

I got this gun used, and was told they that traded it in had over $800.00 into it, so at $350.00 it seemed like a good deal. I am new to glocks and have always heard about their reliability, and that they will eat just about any ammo you feed them, however that was not my experience. I had some older ammo (about 10 yo) so I'm hoping to get out next week with new stuff to make sure that wasn't the issue, but with the 45 rounds that I fired, I had 5 ftfs, and 2 times I had to hit the back of the slide because it didn't go forward all the way. I'm wondering if it has anything to do with the slide that was put on it, or is it just something to be expected out of a glock after it has been shot a bunch. No other history on this gun. Really thinking about taking it back and heading it in towards something else.