This seems to happen more often when the system is running low on memory and I'm resaving big texture files quickly. When memory starts to get low I notice Max takes a while to update textures, if it gets resaved while max is spooling in the new texture, it stops trying to update and pretty much says "ok this file is going…
I've noticed when a single shader has too many textures (like 3 or more usually) it will stop updating. I usually do what unstoppablex recommends or I'll click the little lock symbol (should be to the right of Ambient and Diffuse). I might also toggle hardware shading (the little checker-box to show it up in viewport). All…