Question: problems with assume

Dear Maple Users,

I'm solving quite a complicated task, so I tried to simplified an example.

There is an equation:

SOL := fsolve(Nz+int(int(StrssCctXY(x, y), x = -(1/2)*b .. (1/2)*b), y = -(1/2)*h .. (1/2)*h) = 0, {C1 = -(1/2)*h .. (1/2)*h})

 StrssCctXY(x,y) is piecewise function containing C1 variable, to solve an equation I had to use assumptions on C1 via assume(C1<num1, C1>-num2) command, after that C1 becomes C1~;

Maple is solving an equation and returns an answer C1~=const; everything is fine up to this moment;

Further I need to do some operations with StrssCctXY(x,y) function so I need an expression of function through the x,y; I do as following:

C1 := subs(SOL, C1~);
 StrssCctXY(x,y);

Maple returns expression for the function written through C1~ :

(0.35e-2*(.9848077530*y-.3633316566-.1736481777*x+.9848077530*C1~))/(-.2143263684+.9848077530*C1~)-0.35e-2
 

I can't understand why It doesn't substitute found value of C1 to the function?

I will appreciate any help. Thank you for your attention.

p.s. If needed I can post the original Maple file with the task.