I also wouldn't worry too much. If you're frustrated, though, and want to get stronger, you might want to make sure you're doing several sets. If you're working at the right weight, you should be failing at rep 7-8 on the first set, maybe 6-7 on the next, perhaps 4-5 on the third. You might be able to see progress on how many reps you can do in the third set before you jump weight, and that could help with motivation!