You need to check a few things;
That you have the correct exercise type selected.
For example if you have it set for a squat, but then perform a deadlift then it will miss the first rep every time.
This is because its expecting the bar to be un-racked first then lowered into the squat before the concentric phase is measured on the way back up.
Also you need to be aware of some other key principles.
• Each exercise can have its own minimum movement threshold typically around 10+ inches.
• Double tap on the exercise itself and you will get popup screen that displays its basic setup
• Make sure that this value is not adjusted unreasonably high (Having said that you don't want to set it to say 1 inch ... as then each small movement can be registered as a rep) - so therefore you will get a lot of false additional reps.
You should also note, that just because its set to 10 inches, does not mean that the recording of the movement does not start until after 10 inches. It just means that if the movement fails to continue past the 10 inch threshold then the rep itself will not be counted.
Next there is the 75% rule
If you have a rep that is counted then each subsequent rep needs to be at least 75% the size of the average of the previous reps.
This means that if you do a full ass to grass squat as Rep 1 and then Rep 2 you only perform the movement to half way then this rep wont register. The main thing to be aware of with this rule is that you don't want to record a false rep at the start of the set (say just because your repositioning the bar) and then try an continue to collect data as it will appear that its stopped measurement the reps.
Where as, in fact, the data is still being collected, its just not translating into a valid rep score, because it fails to pass some of the basic rules within the algorithm.