It could be an allergy to the diaper brand. They put lotions and powders in the inside of the diapers, and while some brands may be fine, others are not for particular kids. Once you have a yeast infection, you're prone to them and whatever caused it in the first place.
Something you can do is use Lotriman cream if the ointment doesn't appear to be working. Rx's can get spendy, and Lotriman will do the trick on yeast infections. It can be mixed with diaper rash cream like Balmex or Desitin, or plain milk of magnesia.
You can also AVOID anything with corn in it. That means no corn prodcuts to eat, and no cornstarch baby powder. Yeast thrive on corn. Also keep acidic foods low, so I'd minimize the fruits and acidic veggies, and fruits and veggies that give her loose stools.
Since she's 8 months old, she's old enough for yogurt. Live bacteria cultures in yogurt help tummy troubles, UTI's, and yeast infections. The acidophilus in the yogurt actually eat the yeast.
You could try letting her bottom air out as much as possible. Keep it clean, but don't use commercial wipes. Use paper towels that are damp and after a bath, rinse her with clean water, and if possible, let her go without her diaper for as long as you can, then apply the cream/ointment before putting the diaper back on.
If you find that you're having issues with different diaper brands, I would recommend cloth diapers on top of everything else.