I have bred guppies many times and never had any success with a breeder box. You have to know exactly when she is going to drop (its nearly impossible to tell) because placing her in the box can cause so much stress she usually will abort the pregnancy. Also, just like humans, moving around helps her go into labour. Its best to leave her as stress free as possible (don't move her to another tank) and let her swim around.
The best way to breed them is to have 4 females and 2 males, and many floating plants or hiding places for the fry. Java moss works well and plants along the top are a must. The fry must be separated from the parents and other fish or else most of them will get eaten.
The female may get impatient or nippy with other fish, will often stop eating or will bite food and spit it out, and will sit at the bottom or behind a filter or heater to isolate herself from other fish when she is getting ready to drop the fry. The female will give birth to 1-3 at a time over a period of a few days or a week. The mom will be extremely hungry after so supply food as soon as you know shes given birth.
