Your weapons systems use up supplies each time they are fired. Depending on what your ships are armed with, and what they hit the planet with, they might have just used up all of their fuel/ammo in one go.
Other than that, Emergency Resupply, Supply Storage, and Solar Collectors are your friends.
-Hotfoot
"You know you have a good fight going when both sides run out of supplies halfway through."
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L* GdY $? Fr? C--- S* T? Sf* Tcp- A++ M>M+ Mp!>Mp* RHP!>RV Pw Fq ND? Rp+ G++ MM++