If the limpets don't deploy,
1. you have no limpets
2. your weapons capacitor is empty
3. the limpet controller is switched off
4. a limpet is jammed in the system somewhere (bug happens sometimes when you synthesise 4 limpets, but have less that 4 units of cargo capacity. Solved by abandoning a limpet or more.)
If the limpets deploy but don't collect
1. the cargo hatch is not switched on
2. the cargo scoop is not open
3. the refinery is switched off or missing
4. the refinery is full, or each bin has fragments in, so no room for a new type.
I think we all know that OP forgot to stock up with limpets before setting off on his mining trip.