The problem your having is you didn't do a factory reset before you plugged the LiPo in. I know because the same thing happens to me when i switch from a 9.6v batt to a 7.2v batt.
How it works, is the old TM's (the one you have), detect low batteries by a percentage of input voltage. The input voltage of a 7.2v lipo is actually 8.4v, 80% of that is 6.72v where the triggermaster will tell you your battery is low and stop the gun from firing. That's good since under 6v your LiPo is over-discharged.
NOW, one would ponder, well what happens if you plug in a NEAR DEAD LiPo? Say one at 7v? Wouldn't the cutoff be well under 6v? Well that why the TM has a memory. It remembers that you use 7.2v batteries with an input voltage of 8.4v, so it's naturally set cutoff voltage will be at 6.7v. This is specifically to prevent overdischarge from plugging in a half dead LiPo.
So YOUR issue is, because your used to using 9.6v batts, with an input voltage of around 10.5v, the TM's cutoff is 8.4v. Therefore as soon as you plug in your fully charged 7.2v LiPo the triggermaster thinks your battery is already dead!
So the solution is to do a factory reset, and use your 7.2v LiPo to setup the TM. Typically if you plug in a 9.6v battery afterwards, you have to do another factory reset to be able to use a 7.2v LiPo
