wxMaxima cannot reduce system to a polynomial in one variable

I'm attempting to use wxMaxima 11.04.0 to solve some non-linear 
equations symbolically, but I get the error "algsys: tried and failed 
to reduce system to a polynomial in one variable; give up.". Having 
solved these eqns by hand, I believe there is a solution. Since I am 
new to wxMaxima, I am probably using it incorrectly. Can someone 
suggest how I can get the solution I want?

Here is the wxMaxima transcript: (%i1) A14:d=sr/(DM); A11:\alpha \eta 
*(M(1-d)/\rho)^(1/3)D=r/F; (%o1) d=(rs)/(DM) (%o2) 
(alpha(1-d)^(1/3)etaD*M^(1/3))/rho^(1/3)=r/F [this is actually 
rendered with Greek letters]

(%i3) solve([%o1,%o2],[d,D]); algsys: tried and failed to reduce 
system to a polynomial in one variable; give up.

Substituting d from A14 into A15 yields a cubic in D. I would like to 
see that, and also the solution of that cubic. For values of the 
parameters s, r, etc that I will use, there will be only one real 
root, but of course 'solve' can't know that, as I have asked for a 
symbolic solution.