Any photos of your current installation?

Many doors have the long hollow rubber balloon seal on the bottom. If it is not sealing due to any imperfections in the floor, mark the area. Poke a hole in it and spray in some expanding foam through the just created hole to expand the hollow 'tube'.

It should not take much, but be sure to close the door right after you take your finger off the trigger of the foam gun. Let it sit for at least 20 minutes.

In very windy areas, doubling up on the side and top weatherstrips can add a second layer of protection.

The vinyl ones with the longer flexible lips work great.

They do expand and contract in the heat and cold though, so I always caulk the bottoms where they meet the concrete.