Yeah, sometimes that will happen to me. I usually wait a little while before I try again and it seems to work. It leads me to believe it's a problem on Valve's end.
ive got tat in cs:s a few times
but it doesn't show up on ur fav in-game but wen u go to ur steam "thing" and on the bottom it has servers it shows up there(well at least i do) but its doesn't show up in the in0game thing
Its happened to me before to fix it I went down to my system tray/right click steam icon/servers..they should all be there and join through it. After a couples times they should
reappear.
I've got that in TF2, there might have been more civilized solutions to the problem but I just kept hitting that "Add server to favorites" button until it realized what I wanted it to do.
I've recently had a problem with it adding my OLD fav servers... like from when i started many moons ago... but yes, i've had the said problem happen to me too recently