Psuedolithos Miguirtinus Is Feeling Unwell

The photo below shows the problem. The wall of the cactus is collapsing. The plant shows no sign of rot and is not mushy.
From my limited knowledge we have 3 basic components - soil - water - light. The soil is a good quality potting mixture with 1/3 sand - charcoal and decaying leaf litter.
I only water every 4/6 weeks and the plant is under a wide eave on the western side of the house and probably receives say 3 hours of sun each afternoon and is protected from rain. I live on the Gold Coast.
I increased watering recently just in case that was the problem but it didn't seem to make any difference. I had a look at the roots and there was no sign of any new root growth.
I hate to lose plants but I fear if I don't do something it's on its way to cacti heaven.
Comments appreciated.

Have you checked the roots yet?

Thank you for your reply. I quote from my original post

" I had a look at the roots and there was no sign of any new root growth."

I've had Psuedo's look a bit like this, the young ones can get stressed.

I would re-pot it asap, I'd venture to say it's not liking what it's in greatly. To my eye it looks a bit too organic - too much pine bark (I don't like that in my mixes unless it's composted down as it's supposedly inhibits growth). And are those white spheres slow release fertiliser ? Might be a bit much for such a small plant and if it is fert, could be burning off the roots.

Once repotted, top dress with gravel or something as I find this leads to a much more stable environment for the plant (adjust your watering if necessary) Then put it in a shady spot and give it a day or two before watering.

Psuedo's usually kark it by going to mush very quickly, so I think there's still a chance...

Firstly thank you for your time in replying.
Would you mind giving me some idea as to a suitable potting mixture for this plant.
Yes it is slow release fertiliser ( NPK 13 / 5.7 / 10.8 ) that you see.

Yeah they can be a bit touchy.

I had one for about 6 years in just about the same pot, i re potted it and about a month or 2 later it started to rot, and completely lost it.

A quick drying potting medium that maintains lots of air pockets to keep aerobic conditions at the root zone.

Thank you for your reply.
I would like to think that my potting mixture is as suggested by you. I will keep the mixture slightly damp and maybe water with a weak solution of Seasol ???

What kind of sand are you using? If it's too fine then it clogs up the air spaces and if it's too coarse then it can retain water by reducing contact with the ground, stopping water from flowing out of the soil. Water likes to flow directly from one source to another so contact between the potting soil and the ground below the pot through the drainage holes can improve drainage.

For a sand-based soil (should you choose one - there are plenty of other options) I would recommend washed river sand with its mix of small ~0.5 mm (but not too small because it's washed) particles through to large ~2 mm particles and small gravel particles. Then you can add some organic soil to this and experiment on the proportions of sand and organic soil.

I usually use between 10% and 30% sand depending on the species but since I have not grown any of this genus I cannot recommend proportions. Also remember sand has no nutrients so you would have to add these accordingly if you choose to add sand o your soils.

It's a coarse washed river sand mixed with potting mixture / charcoal / decaying leaf litter.

It's with a heavy heart that I have to inform everyone that I turned off the life support system this morning and the plant is now part of that stunning collection in cacti heaven.

Thanks to everyone for their comments / advice which is appreciated.

Regards,
Wayne