The tightness of the bead & ensuring it makes it to the center of the well has already been mentioned several times.
On occasion, the stiffness of the sidewalls of the tire can be so stiff so as to be the problem. I've had this happen mainly on heavy duty tires that were very deformed & smashed flat during shipping where the ridiculously thick sidewalls sat compressed under many other tires for months on end until a customer bought it. The fortunate thing is that once mounted & inflated the tire takes "a set" to be round & consistant properly from there on out.
In any case, have a seat, get the first bead on. Then work the second bead around until you can't go any more. Then lock your progress with one tire lever & hold it with your knee. Hold the other end of the bead firmly against the rim with your thumb & use the second tire lever spoon side to the rim to lever the bead over the rim. Just be careful not to pinch the tube. This may take several attempts & re-trys of pinching & draging the tire to get the second bead well set in the well while levering the second lever. Finishing at the valve stem ensures the valve stem doesn't take precious mm's of needed space.
I find Pedros levers are the best & the profile at the spoon end tends to avoid pinching the tube better than other brands I have dealt with.
It is 100% technique...& strong thumbs. You'll get it.
