As in many things, the late Sheldon Brown has the answer:

"If your tire dips inward at one spot, it is usually a sign of an unusually tight fit. This may make it a bit of a struggle to install the tire, but it also means that you can get away with considerable overinflation with no risk of blowing the tire off the rim. Indeed, the best way to seat a "dipping" tire is by temporariliy overinflating it until it "pops" into position."

I often have this problem and this advice did the trick.

Thanks, Sheldon.