Question: Why wont my expression substitute?

I am trying to do some simple variable substitution, but for some reason Maple will not do/allow it. I am extremely puzzled by this, and the only workaround seems to be doing it by hand or changing variable names. Any thoughts would be appreciated. For some reason I cannot upload contents...

restart;
T=R/(2*Pi*l^2) - diff(C(R, Q, beta, c), R)/(4*Pi)
                              d                  
                             --- C(R, Q, beta, c)
                      R       dR                 
               T = ------- - --------------------
                         2           4 Pi        
                   2 Pi l                        

P = 1/(8*Pi*l^2)
                                 1   
                          P = -------
                                    2
                              8 Pi l 

isolate((2),l^2)
                           2     1   
                          l  = ------
                               8 P Pi

Want to replace l^2 in (1) with this expression (3), but won't allow it. 
subs((3),(1))
                              d                  
                             --- C(R, Q, beta, c)
                      R       dR                 
               T = ------- - --------------------
                         2           4 Pi        
                   2 Pi l                        

subs_error.mw