I've done three of the 21" front wheels mentioned and each one was a b***d to get the tyre on straight. In the end, it was a case of persistence, at each try forcing the non-alignment around the rim until eventually the tyre popped into place. It's probably unlikely it will right itself. Someone with the same problem as me (same bike) said he sorted it by inflating the tyre to 90 psi but I wouldn't recommend this!!