2 things you should be aware of when capturing. FCP always looks for a time code to capture. It is therefore a good idea to 'time-code' your tape before shooting your footage. This will write (engrave) a time code on your tape. When you are shooting, your footage will then correspond to this time code. Cool, but that's not all. Now as we are capturing footage, FCP needs to know a 'pre-roll' duration, which means when you set an in-point, how far back (of this in-point) should FCP set itself to roll. This is important so that when the tape rolls, your capture is at normal speed and therefore the capture process will be perfect. Now, if you have specified in the setting to have this duration to be 5 sec, for example, FCP will take your in-point position and subtracts 5 secs from it using the time code. It will then start running and capturing when your specified time code appears. But to do so, FCP has to be able to read the time code - 5 secs. If you are at the very beginning of your tape this can cause problems, so you have to either select a 'later' in-point or set the 'pre-roll' duration to a short duration. Mind you, a shorter duration may not be ideal as the camera rolling needs to get into gear first. Anyway, I hope you get the point. Now you see why time-coding your tape initially is good idea (there is other good reasons for it too, but I won't tell all the secrets in one post :-)) and also checking your pre-roll and post roll setting is important too in the capturing process.
Sorry for the lengthy reply, but I hope it is helpful.