Snoods too long
If the snood is longer than the gap to the dropper above, the hook can wrap around the bodyline mid-cast and you'll reel in a tangle. Keep snoods to no more than two-thirds of the dropper spacing.
Crimps not tight
The stop-beads need to clamp firmly either side of each dropper swivel. If they slide, the swivel walks down the bodyline under load and the snoods bunch together. Use a crimping tool, not pliers, and pull-test before fishing.
Wrong hook size
Whiting and dabs have small mouths, a 4/0 hook here will see a lot of taps and very few connections. Match hook to target: size 2 for dabs, size 1 to 1/0 for whiting, 2/0 for codling.
Bait too big
The flapper is a finesse rig. Hanging a whole squid off each snood kills the cast and tangles everything. Save big baits for a single-hook rig and keep flapper baits compact, a single worm with a tip, not a whole shellfish stack.
Not changing rigs over rough ground
Two hooks, two snoods, two chances of a snag. The moment you start losing tackle on every cast, switch to a pulley, losing one hook on a flapper means losing the lead too, which means losing the fish.