The pull is bigger than the mash it comes out of.
The pull does not fit the pot it would be boiled in.
Boiling mash is not hot enough to reach the target rest.
The requested pull was thicker than grain can be drained.
A grain-free pull wants more free liquid than the mash holds.
More than half the mash, by heat capacity, is in the pot.
The boil would evaporate the pull dry.
The schedule's evaporation leaves the mash too stiff.
A thick pull will scorch if it is not stirred.
The schedule is a long day.
Problems with a decoction, plus notes the brewer should see.