It happens

just let them suffer till you message them back :D

Happens to me a lot mostly when ive been using it in map load times and while in the team choose screen. My suggest before cliking shift tab after joining a server if you click escape this usually means you can shift tab after choosing your team. I also Alt Tab and send 1 message through the window on my desktop then go back in and its fixed.

Thumbs up if it helped.

Rippy :]

Try changing the hotkeys in the steam settings.

Try change it to "Home" hotkey, I like it and it's easy to remember :D

Wait for a while after starting up a game... until it say the little thing in the bottom right corner, "use shift tab for blah blah blah"

or just wait for a bit after you press it.