I have an application that has a number of complex rules. These work at present. If I add other rules to the cube, these rules do not function correctly (even though the new rules have no effect on the old ones). Does anybody know why this could be happening ? In addition, there have been instances where the rule works using 1 MDX query (via MDXSample) and fails with another query on the same cell. Any ideas would be gratefully appreciated.

Have you considered Solve Order issues?
If you haven't , you might want to read "Understanding Pass Order and Solve Order" in BOL.
HTH,
Brian Altmann
www.geocities.com/brianaltmann/olap.html

No it is nothing to do with sort order issues (though there were some in the past !) and it does not alter the fact that in some queries it works (but not consistently)

Multidimensional formulas can apply to any member or instance of any
dimension in a model. Because there can exist one formula per member per
dimension, frequently more than one formula applies to a particular cell. In
these cases a precedence rule needs to be invoked to determine which formula
(or ordering of formulas) will be evaluated for the cell.
"Rupert Cavendish" <anonymous (AT) discussions (DOT) microsoft.com> wrote

correctly (even though the new rules have no effect on the old ones). Does
anybody know why this could be happening ? In addition, there have been
instances where the rule works using 1 MDX query (via MDXSample) and fails
with another query on the same cell. Any ideas would be gratefully
appreciated.