Not seeing what you have, I am only speculating here. I do know a #12 trips with no problem-- been a long long time since I've used a #8 so I'm not for sure. Here are 2 possiblilities (and I am assuming you are using a Tiagra).
Are you sure you have 100% free spool when backed all the way off?. Even a tad bid of drag (i.e., if drag is too tight at strike) will not let the planer trip and the reel's spool does not rotate enough to let the planer truely free fall the way it needs to. Another possibility ( but I don't know for sure), is that the #8 is too light to counter act the effects of the clicker and thereby not allowing the planer to freefall. Lastly, When I first started using this system several years ago, I would sometimes have problems with a #16 not tripping. I had the spool totally loaded with line at the time. I reduced it to half spool and also realized I must have total free spool so made sure of this. Point being something fixed the problem. The only time my planer doesn't trip every time is in a big head sea, and sometimes the waves do something to the boat that prevents the trip. The only other thing I can think of is that you may not be giving enough time in free spool before locking back up. If done properly (with a #16 or bigger), you will swear you are getting a backlash--- let it work itself out and THEN put it back in gear.
Just for shits & giggles I would give a #12 or bigger a try.
For the Floridian that don't know dixie, 31" is used because it is the proper length to match the planer and allow the right amount of space to trip but not too much to tangle or cause unecessary resistance in the water. There are several videos on this bridle system. I don't mean to speak for anyone else, but I can tell you it is the easiest, most effective way to use a planer I have ever seen. It's a meat stick so the buggy whippers and ultra light guys may poo poo it. No denying the fact that there are more "sporting" ways to catch a fish, but I tell ya one thing, when you have a 40 - 50lb Wahoo on the business end and you grab that big ole planer with one hand and it is tugging for all it worth--- that is some kinfd of fun.
