Preheat as much as possible. What you fight when welding aluminum with a stick arc is temperature. Aluminum is like most non-ferrous metals, they try to be all the same temperature at once. That makes for what you found, cold welds, poor penetration and such. Aluminum rods burn fast enough without needing to use the flame from them to heat the mass of what you are trying to weld.
When possible to aid preheating, have enough kitty litter(oil sorb) to bury most of the parts in then preheat them and let them cool in the kitty litter. Works well with cast iron also.
Welding aluminum in open air is work!