netlist error (invalid subcircuit)

Hello,
Im trying to simulate an adder circuit which produces an output depending on an input offset. That is the offset gets added to a temp variable and whenever the addition results in an overflow it should produce an output.
The adder is a 4 bit adder and so whener the sum exceeds 1111 it shud produce a carry out and the resulting sum shud  substract 16 and produce a new sum which would then be smaller than 1111. I hope u get the point.
i made the TB for the circuit which consists of a simple 4 bit Ripple carry adder and a register driven by the clock because I want the addition to take place with the clock. The out put of the adder feeds the register and the reg op  feeds one of the inputs to the adder thus creating a loop structure.
bt whenever i try to simulate the Test bench i get this strange error in the spectre.out file which says that "so and so instance is an instance of an undefined model subcircuit" . Here 'so and so' refers to the instance of the my module which contains all the logic to do what i wish to do as said above. The symbol of that schematic that is.
This strange error disappears when i make an exact copy of the same Test Bench into another schematic. And again after a while the same error pops up and then i recreate another schematic and then it works..which is becoming kind of annoying. Does anyone have any pointers as to why is it happening?
Hope to get some hints about that.