This is a very common question. Honestly, I wouldn't be worried if your team couldn't consistently complete even one mission. My first year, we had one or two missions that worked about fifty percent of the time. When we got to the tournament, neither of them worked at all. The kids quickly recognized that they had some work to do. We had our practice round and our first official round and had scored a total of zero points. We actually had negative points, but for FLL the lowest score assigned is zero. Anyway, the kids had started seeing what some other teams were doing that was similar to what we were doing, only the other teams were a lot more consistent. They were able to apply some of what they learned from the other team to their program. Furthermore, we picked up one or two other missions, just as a result of seeing other teams. One mission in particular required driving all the way across the table, spinning around, threading a needle with mittens on, all while hopping on one foot and balancing a stack of plates. Ok, maybe it wasn't *THAT* hard, but to the team it was. In fact, no other team at the tournament had even attempted it. But my team wanted to do it. By the end of the day, we were doing that mission consistently on the practice table, but we hadn't been able to succeed on the official table. Well, for the last run, they tried it and got it! The place exploded! We didn't get the highest score that day, but it didn't matter. In fact, it didn't matter to me that they were able to do it on the official table at all. I had seen them do it on the practice table and that was good enough to me. But of course, I really hoped it would work for them.
That's a long story to say, go to the tournament, no matter how good you think your team is. I think they will impress you and themselves. It's worth it. I hope that you do.