Glad I saw this thread as I've been having similar issues but not consistently. My stout and brown ale were fine, good drainage when the basket was lifted and fine sparge.
The braggot I made was wonky but I think it was my own fault trying to use a tea strainer instead of a sink strainer, it instantly blocked and the pump ran dry which probably compacted the bed, but I still managed 85% efficiency somehow.
The last brew was a simple pale ale, 3.5kg of maris otter and 500g of munich, circ'd without any strainer and when I lifted it the run through was pretty slow, so I stirred the mash and it was very solid, had to stir a 2nd time mid sparge as well. Also as I used the regular pipework for a 4kg grain bill I had to add an extra 2L water so the to plate would seal so it wasn't the thickest of mashes. I also seem to get pretty murky wort so I'm thinking that I'm not getting the flow through the grain I should.
Going to try bringing the flow up gradually as recommended here, also a mid-mash stir might be in order to check and see how the grain bed is, also has the benefit of helping keep efficiency high.