Drill PSR- 28 LI-2 not working with new batteries

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Joinery installation in the broadest sense
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This is odd: PSR 18 Li-2 wokrks with the 1.5Ah batteries but not with the 2.5 Ah batteries. not sure if the issue is with the newer "alliance" model. I get a brief flash of the Power Light and nothing else. 

Both batteries work on fine on PSB 18 LI-2 
1.5 Ah does not work on a trimmer (too little juice or not sufficient cooling?)

Any idea why / what's the issue? Any way to overcome (I use the PSR a lot more than PSB due to the size -- now stuck as want to use fewer batteries with more capacity)
not quite the promised ecosystem of tools that all work on the single battery system :-( 

The setup on the photo wrks - but not if I swap batteries. 
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Other building trade
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Same problem for a PST 18 LI
Works on 1.5 amp batteries but not 2.5 amp ones. All other tools I have are fine and batteries are new/like new.

I will take it back for a refund if it doesn't work and there is no solution.