Well, both strap and buckle (rep or gen, doesn't matter) should be 18mm - just take caliper, measure both, and you will know what the problem is.
Just to mention - there are two versions of rep kevlar straps for Top Gun - one came out on fist version, and another on "V2". There is huge difference between them, strap from "V2" is almost gen like (fell, softness, quality), and first version is pure rubbish - I have both, so I speak from experience.
regards