Brachypelma on the whole is a slow growing genus. Taking many months between molts and sometimes a year+ is not unheard of.
We do not have that species, but we do have a B. smithi, B. emilia and a B. vagans. The B. vagans is the only one we have had since that size and it molted about once a month until it got to about 3/4" and now it has been months since it has molted. During those first months though, it was kept in an un-air-conditioned room during a humid Indiana summer, which gave way to months of 64-69F days this winter. It was fed 1(maybe 2) B. lateralis once a week or two.
The growth of your spiderling depends on the temps. that it is kept at and the amount of food it is given. If yours is taking forever to molt, then you can up both and hurry the process along. There is even a husbandry style called powerfeeding for spiderlings to make them grow faster. An advanced search, with the word powerfeeding in the tarantula section should yield you information on it.
i keep all my slings in a humid environment ~80%, with 84F average temps and feed them 2-4 times a week, in my expirience all of my slings molted in 30 days or so even the rumored slow growers like my B.smithy which is getting a bit chunky >.<