Mortar failing in 2 year old garden wall

[Knobbly]

I put this wall up a couple of years ago. Comprises 'Mayfair Yellow' handmade bricks topped by engineering bricks. Mortar mix was 4:1 soft sand to cement.
Bricks, sand and cement were purchased 'new' from a local Builders Merchants. Cement was Bluecircle Mastercrete (yellow/blue plastic bag version), and was well in date.

The mortar between the handmade bricks has completely failed and is crumbling out like sand. The same mix between the engineering bricks is perfect - only crackinig because the supporting bricks are not stable.
When I put the wall up I was aware that the handmade bricks were drawing the moisture away from the mortar, so I soaked them before laying; doing this the mortar appeared to set at the normal speed and appeared to fully harden over the course of several days

What have I done wrong?

Cheers

 

[Old Salt]

With it being on a corner I suspect it may be down to frost. Not a brickie though so maybe take that with a pinch of salt.

 

[Charlie George]

It does look like frost damage, what time of year was it built? the brick on edge could have been built on a warmer day, hence them being ok.

 

[Alastairreid]

Is it a retaining wall?

 

[Knobbly]

Thanks for the replies. I built this in summer 2021. There were no frosts for at least 6 months after it went up.
Yes - it is a retaining wall in places, but the mortar has failed where it is - and isn't retaining. Behind the visible brickwork is a concrete block wall (I put this up at the same time). The brickwork is tied into the concrete blocks using stainless steel ties.

Here's some more of the wall which is also failing - see the mortar on the ground which has fallen out

I know my bricklaying is not brilliant, but can't work out why they mortar between the handmade bricks has failed.

 

[tony1851]

To me, it looks as though the mortar has dried out too quickly, even though you wet the bricks first; they may have been particularly thirsty.

Did you add any plasticiser? This helps retain water within the mortar so that it does not get absorbed by the brick too quickly. It also makes the mortar easier to work.

PS just noticed you used Mastercrete - I think that already has a plasticiser in it, but I might be wrong.

 

[Knobbly]

Quite possibly, it was during a hot period.

Yes - I used plasticiser.

 

[tony1851]

If there's already plasticiser in Mastercrete, could it be that you've added too much additional plasticiser, which weakens the mortar?

If so, you seem to have included a yellow/ochre additive?

 

[DiyNutJob]

Maybe those bricks took more water than what you soaked them with?

 

[DiyNutJob]

After dry you could wet the mortar again to strengthen it. This might be what you need.