Failing/flaking mortar

[pls1]

A couple of months ago I repointed a sandstone wall with a lime/cement based mortar (mix 5:1:1 sand:cement:hydrated lime).
Since we've had a period of heavy rain and the wall has got damp/wet, I've noticed that some of the mortar has begun to 'flake' on the surface and drop away. It looks quite unsightly so is there anything I can do to stop it & why is it occuring?

 

[noseall]

was the wall very dry when pointed?

if so it is probably the suction in the sandstone drying out the mortar too rapidly.

you should wet the area to be pointed first, allowing it to dry slightly before pointing, but it should not be bone dry.

 

[pls1]

No, if anything it was damp in places since its a retaining wall.
Only reason I can think of was the hydrated lime had been sitting around for a while. It was still dry and powdery, not lumpy though.

 

[noseall]

was the weather a little chilly on the evening you pointed the wall?

 

[^woody^]

I doubt its the lime specifically.

Two other possibilities are that the mortar was not uniformially mixed, or the joint was not sufficiently troweled/compacted to give it its important weathering finish

And two months ago may have had some overnight frosts or freezing temps