Welcome to the forum.
A question is how are these jets plumbed into the return system? Are all 5 returns plumbed from a common point with the splits underground or are some of the jets pipes brought to the surface and the pipes connected with valves to the filter output? Can't imagine anything getting into the return plumbing to plug something. If a pipe got crushed, for example, it could cause the problem but then you would have a pretty big leak.
Al