Question: simplify(%) don't simplify

Good morning.

I've this funny problem with maple11. I get an expression as output from a calculus, and I try to simplify it with simplify(%), but simplify don't simplify and give the same expression as result.

On the other hand, If I copy the expression and paste it as argument of simplify it work fine.

Anyone know why this happens?

Thank you

S.

ps: the expression to simplify (in fact is more simple than a simplification: there are terms equals but with different sign to cancel togheter) is

(1/4)*(diff(mu3(r), r))^2*exp(2*mu3(r))+(1/2)*(diff(mu3(r), r))*exp(mu3(r)+psi(r))*sin(theta)*(diff(psi(r), r))+exp(mu2(r))+(3/4)*(diff(mu3(r), r))^2*exp(2*mu3(r))*cos(theta)^2-(1/4)*(diff(mu3(r), r))^2*exp(2*mu3(r))-(3/4)*(diff(mu3(r), r))^2*exp(2*mu3(r))*cos(theta)^2-(1/2)*(diff(mu3(r), r))*exp(mu3(r)+psi(r))*sin(theta)*(diff(psi(r), r))