Mine work fine but they're not used professionally - just as hobby DIY tools.
The tools have three contact strips - Plus & Minus 25 mm long and a third 3 mm connector that must I suppose act as a safety interlock - the ON switch.
My guess is that this short contact is the one that's causing your problem - not always making a connection.
You might try to eliminate the culprit - is it the battery pack or is it the tool? Do you have other tools that work OK or other batteries that do not give problems.
I am convinced based on what you say that there's a failure of the interlock connector - not making a good connection - might it be corroded? You might clean it a bit with sandpaper.
Good Luck...
April 2016