[Barrett]

Reinout makes a good point. Another instance: at a Champlin Museum symposium c. 1990, WW II ace Walter Krupinski said that there was surprisingly little air-air gunnery practice in the prewar Luftwaffe. First time he shot air-air was at an Avro Anson over the channel in 1940; he and his leader "fired out" (went dry) and the Anson motored off. I forget the exact figures, but WK had something like 100 missions before his first kill, and perhaps 100-150 more before he got No. 10. Then came Barbarosa and things quickly got hot.
A friend of mine, 5th AF P-38 type, had two ambitions: Shoot down a Japanese airplane and make captain. (Well, OK, he wanted to go home and marry Lizzie, too.) He got one victory in his first 5 months of combat, them BAM: in December '44 he led V Fighter Command with 6 kills in 3 fights, outscoring Bong, McGuire, and everybody for that period.
Sometimes it just takes awhile for circumstances, experience, and the Moon in Jupiter to line up.