The only reason I can think of that you couldn't jump start would be that you have too much current draw for your cables.

When I was having glow plug problems, it was that my glow plug controller/relay would stay on for 3 minutes, no matter what the light said it was doing. When I tried to crank with it doing that, I would have very slow cranking, even with an almost freshly charged battery.

Once I fixed the relay (in my case, it was spark eroded points on the relay inside), it would turn off when I turned the ignition key to crank, and the car cranked up much more energetically. Jump starting has been no problem since then.

This issue with my controller also pointed out the fact that the controller can pull the better part of 80A, whereas the alternator only puts out somewhere around 74A at peak output. (and looking at the specs, it looks like peak alternator output is somewhere around 4k engine rpm. I drive around 1.5k...) Fixing the controller issue helped my dead battery problem as well.

I don't know how much this helps, but I suspect that your glow plug controller is staying on when it should be turning off.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...