My 13-yr old daughter had her first period 6 weeks ago but nothing since. why?

Because she's not: Ovulating yet. She needs to carry "emergency supplies" with her at all times, because her periods will be unpredictable for quite a while.

Early cycles: Young women often do not ovulate much the first year of menses but the estrogen driving the changes will bring on cycles. Irregular flow patterns from 2 to 10 weeks are occasionally seen. Once they begin to ovulate on a regular basis a 2nd hormone sets the clock.