Question: Has doesn't work sometimes

Hi

I work with rather long expressions, expecially fractions containing "good" functions, like power or sqrt.
They contain also some contants, over whick I made assumptions like

assume(M::real); additionally(M::positive):

It happens very frequently that when I go further in my calculations, doing simplifications for ex, I get finally expressions which still contain the constant M, but for which

has(%, M)

answers false.
It happens also that these expressions contain terms like 6*M~ + 2*M~ which maple doesn't simpify, like if the two M were two different constants.

I really don't understand why this happens, but it's very unpleseant because I can't have fully simplified expressions, and I can't use subs(M=xxx,%)

Thank you for any help

S.