B.smithi's aren't really big on burrowing anyway....Like lopez said, it can take them ages to decide to burrow if at all. i have a B. smithi who has gone for two months without burrowing. one molt cycle for me and it still hasn't burrowed. they will typically do it when they are ready, if they want to.
My 3.5" took just over a month and then made a lovely burrow over a weekend.As adults, it can take months for them to decide to burrow in a new tank.
I'd suggest reading over the breeding report forums.iv been keeping tarantulas for about 3 and a half years just never bred them so fancy having a go now, just making sure on everything before i go into it