2.25 tyres

[popcorn_skollie]

Are the tires new.

If not could it not be a snapped/broken bead that is loose in the tire rubber casing that has stretched making the inner diameter of the bead bigger. My 2 cents.

Also possible, if you still struggling. Nobody better than droo to have a look

[dirtypot]

Just make sure that the rest of the bead (that is already inside) is sitting in the middle (deepest part) of the wheel well.  That will allow you the most slack to get the last part over the edge of the rim.  If it's resting on the edge then you're going to struggle.