Unless the items have been hacked it works for all types. However, if you have two items readied for your feet, or another item which it might not stack with magically, that might be the problem.
Also, it might be possible that dispel magic removed the speed temporarily from the boots. Try unreadying them and then readying them again.
Wizards often have encumbrance problems, especially if carrying loads of darts.
If nothing works check to see if the items have been alterred. Boots of Speed are Items.dat record 45.