New kittens less then 8 wks

Question
Hi we have just got 2 new kittens both sisters at 6 weeks of age... They are eating dry food great and are very good besides there play fighting that we have been breaking up..
The one kitten has been litter fully trained or at least fine with it...  the other kitten is pooing in it but not peeing for some reason, we have been putting her in it randomly
maybe not regularly .. she was peeing on a pile of clothes and we removed it, now we found on the bed she has been peeing needless to say we would love to correct this as she will only be turning 8 weeks in 2 days now and the sooner we figure the better.
They have been vet checked 2-3 days after we got them
dewormed first shot and flea treated (we would have asked the vet but she was too new to notice yet at that point)
Any suggestions
I was told it definitely is not a territorial thing as they are so young...

Answer
Lisa,

She is a little young but it is possible that she may have a urinary tract infection. It is painful when they pee and they associate the pain with the litterbox and go elsewhere. I would have the vet check her for that.

Try using a puppy pee pad next to the litterbox. Some kittens do not like the feel of litter when they pee for some strange reason.

Also keep more litterboxes available. Kittens don't go far when they are looking for a litterbox.

I hope this helps.

Tabbi