The clip (tank) should be pressed at its lower part - the part nearer to the wheels. If you notice the upper part is slightly sunken thats because the clip is underneath it so it would lock onto the thigh part of Prime. NO need for excessive force on this, as soon as the clip is up - keep it pressed,then try to pull down on the leg. Some have made the mistake of just pushing it then pulling - it wont slide the leg down that way. You need to keep it pressed then pull on it. Also when you need to transform him back you need to keep pressing on the tank before you can slide the legs back up, coz its the tank that keeps the leg from sliding freely and lock it into its place in bot bot and truck mode. Hope this helps.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]