Impedence Matching Possibly?

[modsquad]

When I connect my BSIABII, Umble, Hwy 89 in series I notice that the order of the effects in the chain makes a big difference on the final output of the signal.  For example when I have Guiter-->HWY 89-->Umble-->BSIAB-->Amp and switch on the BSIAB the signal output is less than Guitar-->BSIAB-->Amp combination.  Is this caused by mismatched impedence or something else.  Should I put a buffer between the effects?  Its really pissing me off at this point though.

Stan