It's probably undissolved powder so running it through with some
pre-mixed washing solution & washing soda at max temperature on Cotton
wash should fix it. & yes, clean out the dispensers if necessary.

We have similar problems especially since starting to use low temp
washes. The other thing to look at is inside the seal, we regularly
have to clean this out, peeling it right back to wipe the edge of the
drum.
A regular boil wash really helps as well.