1. Don't use something like Tide with Downy. The fabric softener will affect absorbency.
2. Some detergents out there just aren't that great at cleaning (Dreft, for example) and aren't strong enough to actually get diapers clean.
ETA: This is not to say that the only detergents that may cause problems with PFs & flats are Dreft and Tide with Downy. The hardness of your water also plays a big role in how well a detergent works.
Re: quick question
1. Don't use something like Tide with Downy. The fabric softener will affect absorbency.
2. Some detergents out there just aren't that great at cleaning (Dreft, for example) and aren't strong enough to actually get diapers clean.
ETA: This is not to say that the only detergents that may cause problems with PFs & flats are Dreft and Tide with Downy. The hardness of your water also plays a big role in how well a detergent works.