I am delighted that the gun is working again....it is possible that a pellet may have jammed up somewhere and that caused a problem. I forgot to tell you..but you should NOT be using the Heng Long BBs that come with the tank as they are of a very poor quality and can actually damage the gun!!
You need to go to your local store and buy some 'Precision Ground' 6mm BBs....and apparently you should NOT re-use them once they have been fired due to the risk of introducing dirt or dust into the gun. It is also possible for a pellet to have been chipped and for this to then tear/ rip the rubber seal !! ?? !!
Please ask away about the rear idler in a new thread, but I expect that the idler is not at right angles to the hull and maybe it is getting close the throwing a track....so you will need to buy / make a track tensioner adjuster kit or a strengthening bar to run between the two idlers....am I close

Alb.