Hi there,
Take a read of the first post in Please read: make it easier to help you (Don’t worry about the long thread).
In particular, it’s easier to help if you can simplify your problem to a minimal working example. Since we can’t run your example, it’s a little hard to offer advice.
The most common reason for NLopt to return :FORCED_STOP is an error in the objective function (NLopt Reference - NLopt Documentation). You probably need to check that fxp solved correctly before trying to return the likelihood.