Snood too short
If the hook trace is shorter than the lead link, the pulley action is wasted, the lead can't lift clear. Aim for 60–90 cm of snood and keep the lead link to roughly half that.
Weak link too strong
The rotten bottom needs to be the weakest point in the rig. Use a line lighter than your snood (10–15 lb is typical). If it doesn't break before the snood does, you'll lose the fish along with the lead.
Forgetting the bead
Without the bead between the top swivel and the pulley, the pulley can slam into the knot under load and weaken it. A 6 mm rubber or hard plastic bead is enough.
Wrong lead style
A plain bomb lead doesn't grip rough ground well. Use a wired grip lead, when the fish takes and the pulley lifts, the wires fold cleanly and release. With a plain lead you'll skid through every rock on the way in.