Replaced axle on rear wheel. Once I ride it, it tightens and wheel doesn't spin
Yesterday, the axle on my rear wheel snapped. I spent today trying to replace it with a new axle of same length. Before doing so, I checked bearings, and they seemed ok. When I put the hardware on the new axle, the axle spins (although feels a bit gritty), but as soon as I ride it, something tightens and the axle no longer spins, and I need to push the bike, dragging the wheel.
What have I done wrong? ?
1? Is it likely there is damage to the bearings/hub that I haven't spotted? ?
2? Perhaps I have put in too many/too few ball bearings? (I didn't count them so have replaced with what looks right). Should they sit snug, some slightly above the others, or all on same level with slight gaps? ?
3? Have I not used the lock nuts appropriately? Perhaps the cones are still spinning?