You need to get a little bit of cleaner or solvent and spray into the buckle as it probably has sugar or something in it. The solvent will help to release whatever is sticking otherwise you may have to replace it.
Sometimes the locking mechanism will become dry which can cause the seatbelt to stick. The best way to solve this problem would be to purchase some wd-40 to help.
Where seat belt is anchored, undo the bolt. Raise the seat up and down, then up agin to the upright position. Then tighten the bolts and the seat belt should move freely.
