Phew. And I haven’t read any posts above yet, so this might end up sounding late … But …
1. Most important : the straddle wire needs to be different to the wire attached to your brake lever. Those shimano pills are hideous. Awful, etc … is there any more to say? OK, maybe just that it is hilarious that anyone thinks that having used them, it might be a good idea to try and sell them to someone else …Don‘t buy them)
1a: See 1. (This will cut out 90% of shit cantis).
2. Make sure that the cable hanger allows the straddle wire to move easily (if there are little grub screws on the hanger triangle, bin them).
3. If you have observed 1 to 2 above and you still have bad cantilevers life is looking good. Fear not. Just give it a wiggle.
4. what about those hex key tension spring adjusters? If you have observed 1 & 2 above this is prolly not an issue. If it still is, have a go with them and see what happens- chances are you’ll figure it out.
5. Still having problems? Post here with why or message me - cantilevers are cool.