Sometimes it's hard to catch the draft of a fast group even when you're on a fast road bike. You have to pick up speed before they pass you, not afterwards. I've been dropped several times as I've tried to fall to the back of the paceline, especially if the one now pulling decides to pick up the pace a bit.
The mountain bike and wider tires and panniers might make 1-2 mph of difference in speed, but that's enough to drop you from a fast paceline. But keep trying to grab the end of the line if they come by again.
