Question: dsolve/numeric cannot solve if known procedures contain symbolic arguments

Dear bright people of MaplePrimes, 

I'm stuck in a problem with dsolve.

I have a ODE system that I would like to solve numerically (because it's huge) in vars: var1(t), var2(t), var3(t), etc... Inside the ODE there are procedures with arguments like proc(f(x,y), a, b, c, etc...) where f(x,y) is a function for a curve (so x and y are variables) and a,b,c are numeric. 
Procedures have been written as indicated in the help page, i.e. differentiating whether the procedures are called with symbolic arguments or numeric arguments. So I call dsolve specifying the "known" procedures and a numeric method. Maple sets the problem correctly. However, when it tries to solve the equations numerically it points out that in the ODE ys there are some undefined parameters. Specifically, those parameters are x, y. But, again, x and y are not parameters but variables that are used in the procedures within the ODEs. 

I tried to include x and y as parameters and solve the system. However, before retrieving the solution, a numerical value must be given to all the parameters. 

What should I do if I need to keep f(x,y) symbolic in my procedures but I want to solve my ODE numerically? 

Thank you for supporting me. 
Thank you so much. 

Andrea