I've got natural unbleached flannel stapled tight over a wooden framework. Blocks stay put BUT any that are up there longterm (only BOMs BW - don't start nagging!) are held with a couple of pins. They fall off when I start fiddling about a lot with placements on there, for eg with OBWs. I think the space behind is the problem - I might get Hans to put some insulation material behind it when he's got a minute! LOL