Do you mean that the bottom bracket bearings are not spinning freely, or the cranks are dragging on something? Sorry if I'm being obtuse
You want to be careful with the cranks - if they keep coming loose, it's possible that the taper in the crank has becoming deformed, especially if it has been ridden when loose, or over-tightened onto the spindle a few times. If that is the case, a new crankset might be in order, unfortunately... The BB spindle is usually harder than crank, so might still be ok.