You've not got the action set at the right distance from the end of the chassis.
Undo the two cap screws what hold the action to the chassis and set the housing about 1.5/2mm from the end of the chassis,this will be a starting point,it will probably need a few attempts to get it right.
Another way is to undo the 2 allan bolts holding the barrel to the chassis and very slightly move the barrel further into the chassis.
If after doing this it still jumps open you've either got too much tension on the muzzle tube or the cocking leaver has lost its profile where it engages over the cocking roller bearings on the muzzle tube.
You can either re profile it or fit another cocking leaver.
I think someone covered this on here before with pictures showing the area it needs re profiling.