st: permute xtmixed

[Fotios Drenos <rmhafdr@ucl.ac.uk>]

Dear All,
I am trying to run a permutation tests on an xtmixed command and I 
get a number of errors.

I am using the "use http://www.stata-press.com/data/r9/pig"; dataset:

The program for the xtmixed command is:

program test7, rclass
	version 9
	args dependent fac_week
	xi: xtmixed `dependent' i.`fac_week' ||id: `fac_week', cov(unstruct)
	assert e(converged)==1
	testparm _I`fac_week'*
	return scalar pgene=r(p)
end

and then using 0 , 1 , 2 or 3 as seeds I run the command:
permute weight t=r(pgene), reps(2) noi: test7 weight week

For seeds 0 1 and 2 problems arise during the permutation, while seed 
3 seems to work.

I have used the assert e(converged)==1 after the xtmixed and fails 
for the first three seeds.

Is there something that I fail to see? Is there a specific reason 
while the xtmixed command is not converging?

All the best

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