Well considering that the Yak-3 was meant for Dogfighting, and the Yak-9T was meant for a fast armored vehicle buster, the 37mm kinda threw off the Yak-9 because it was a meant for ground attack, and dogfighting with a heavier gun on a light plane didn't really work well.
And there is the problem. In these single missions they want you to use this plane for Need Cover missions or destroying enemy planes etc.