Possible trouble with Malkoff Maglite D 2-3 cell drop in, input sought...

I have a Malkoff drop in for a 2-3 D cell Maglite. When I run it in a 3 cell body with regular alkaline D batteries, it works fine. When I put in a 2 cell body with alkaline or rechargeable Energizer batteries, it will not always start up/light. I can see the LED just barely glowing when on 2 cells. I swapped the switch between on the 2 and 3 cell Maglite bodies, thus eliminating it as the problem. I bought this particular unit because I wanted the versatility to switch between 2-3 cell bodies. The 2 cell body with the rechargeables make a light weight high output unit. I see now Malkoff offers one drop in for the 2 cells, and another for the 3-6 cells. Is 2 D cells not enough power for my drop in?