kitten litter tray problems

Hi all, my partner and I bought a british shorthair colourpoint cat at the beginning of the year (approx 3 at the time of purchase but we havent had it confirmed) anyway new to the home she wasnt using the litter tray we bought her, we tried all sorts - getting another tray, going through different types of litter, moving the trays around etc and finally we got her using the tray. it is currently in the understairs cupboard at the back of the kitchen (opposite the back door) the door to where her tray is doesnt shut properly so is always open. anyway, for the past couple of months she has stopped using her tray and has been pooing on the front door mat. when she originally did this it was a coir matting and was when we were first training her so we got a new different type of mat but she has taken to that too. even when we have taken the mat out she has done her business on the bare wood underneath. we have cleaned the floor and mat with everything we have been suggested online but she is still using the mat. the problem we have is that she does it at night when we are in bed so we cant stop her or try to pick her up and put her in her tray before she does it! it is starting to annoy our housemate and is a little frustrating for my partner and I!!
On sunday night i emptied her tray out of all the litter, cleaned it with an antibacterial spray from the pet shop and refilled with new (same brand etc) litter as before and that evening while we were watching tele she used it twice to poo and one wee and both times i emptied it out straight away and gave her a treat for using the tray. but monday, tuesday she has gone back to pooing on the front door mat! (although monday night she did a wee in the tray)
we are so confused and really dont know what to do with her! we gave her a worming tablet at the weekend in case she had problems there but nothing has changed! last night i wiped the sides of the tray down with the antibac spray and put a think layer of fresh litter on top before bed in the hope it would freshen it up (even though it was clean) but she still used the mat last night.
sorry to ramble on but I've not dealt with this before and its getting very frustrating!

 

Its just a case of doing some training. This is something the mother cat usually teaches the kitten. What age did you get your kitten? Usually 3 months old is the best age to ensure its fully trained by the mother and ready to leave the litter.

I did however have the same issue with 1 of my cats that decided to make the bottom step its poo poo place for a short while “and you can image walking down the stairs in the dark wasn’t very clever”

Anyway firstly keep picking up your cat and showing it where the litter tray is located. Every hour pick her up and place her in the tray. Repeat this 3-5 times and then same again in another hour times.

After a week or so If she poo’s in the same place why not try moving the litter tray near this location.