I agree on the inertia spring issue.
Do this...
Take the stock off of the gun, locate the inertial block spring and inertia block lever spring (#'s 94 & 88 in attached image) - most likely the inertia block spring (94) -chances are one of these is broken. If not broken then clean action well and try again.
If broken... You will need to know the wire diameter of the spring wire, then call Coles - they should have it in stock, but there are different diameters so you will need to know that up front.
Ask to speak with 'tech' and they can explain the springs, how to change them, etc. - It is easy to do, you just need some guidance.