First double and tripple check that the mast step is exacally in the right place. Off by an inch at the base is a huge alignment problem at the top (or midway up).
Once you confirm this is correct, loosen up all the shrouds, and take off the backstay or make sure it is really loose.
If that doesn't work get one side attached, then use a halyard
to pull the mast over to the other side. This shouldn't be necessary, and indicates the rigger made the shrouds to tight, but it isn't worth replacing a shroud