OK, first look at the syst resource. The bottom 3 fields deal with the reinforcement fleets. For testing purposes, set the interval and delay to 0. Interval is the number of days between each reinforcement fleet can be called, and delay is the time (in frames) it takes for the fleet to get there. While you're there, check to see if the correct fleet is in the field.
Next, check the govt resource. Specifically, the Maximum Odds field. For testing purposes, set to 0. This determines how many times their number they will take on, 200 being that they'll go against twice their number of ships without flinching, but above that they call in a reinforcement fleet. Having said that, look at the ship's resource. The strength (left hand column near the bottom in EVNEW) is used when determining combat odds. If it's really high, lower it. Set it to 0 if you want, again for testing purposes.
Now try it. If it doesn't work now, I don't know whats wrong, but if it does, you can tweak the values of the fields I've just mentioned to your satisfaction.