it is more than likely that the spring on the bolt lock is broken, weak and then again it mite be clogged up with dirt. the bolt lock is inside of the bolt with the firing pin running threw the center of it. the bolt lock is designed to hold the bolt closed by springing up-ward into the barrel latch hole holding it.it is a simple fix but if you aren't familiar with it then yes indeed take it to a qualified gun smith. the worst is you will need to replace the hole bolt which last time i looked at them i think they were around 20 bucks. good luck and report back with us. maybe we all will learn something new!!
p.s. there is a great video on you tube explaining how to disassemble a Mossberg 500 you might want to look into that before you go spend money. if you do you will see what i mean about the bolt assembly i had one that was starting to stick so i soaked it ...it was filthy and it works so much better. one more thing DON'T TRY TO DISASSEMBLE THE BOLT if soaking it and cleaning it and oiling it ...take it to shop