Dear Raghava
I would suggest first to shift the engine to QRPEM and stop early when you see no change in -2LL(should be around 1652).
QRPEM is a very good algorithm when you want to get good final estimates but does not really about -2LL converging perfectly(it should oscillate and it is good enough).
I am running FOCE like you did and I do not see any problem other than you should never ask for standard errors before you are sure you at done with the fit.
Then do the following
Copy your model to the workflow
Shift to QRPEM and run it without se request and wait until you see no change in -2LL. Then click on "stop early"
Copy that model to the workflow, ask for 1 iteration and for se's.
It should work.
If still problems, let me know and I will send you the project working.
Now an advise, in general F must be between 0 and 1. It is better to put instead of F , ilogit(flogit) where flogit is a parameter you define as normally distributed(sum option in parameter/structural tab) and not default lognormal. If you do that, you are 100% sure bioavailability will be between 0 and 1
Best Regards
Serge