Cat poops next to litter box

Hi all,

I adopted two rescues 6 years ago (a mum 12 years old and son 10 years old) and ever since have had an issue with the mum pooping next to the litter boxes.

We consistently use catsan ultra clumping litter apart from trying different litters.

We have fed pretty much the entire time felix as good as it looks and she begs us for it and always ears it instantly. We also give purina senior dry cat food but she doesn't like it as much as the son does.

We also have mats down in front and around the boxes and she generally poos on them or the hard wood floor.

Her poos are generally pretty normal and firm.

She wees absolutely fine without fail in the boxes.

Me and my wife have tried the following with absolutely no success.

Different litters (brands and types) I. E. Clumping, non clumping, wood pellet fine and coarse, fragranced ones)

Different foods

Different trays (lidded ones with flaps, corner ones, extra large ones, deep lined ones)

Different tray configurations (1 to 4 in various places - we normally have three)

Feliway sprays,

A litter box with nothing in it.

A litter box with a mat in it.

Cleaning the entire area (washing trays and mats).

It's like she never learnt to poo in a box and still can't. I don't know how to get her to at her age.

She will do it in front of us and will walk in the box, do a wee and then walk out and do a poo.

Once she's pooed on the floor she spends 5 minutes scraping imaginary litter onto it.

It sti ks the house out and the litter boxes are in the study where my wife works from who is pregnant so I worry about toxoplasmosis too.

Any help please????