Repeated issues with exterior render corners

[Sam6970]

Bought a house with a new extension not too long ago. The render on the extension had chipped off in the bottom corners. We got the person who did the work to come back (with some difficulty) and repair it but then it just chipped off again. I’ve tried three times to fix it now with only limited success.

I have no real expertise my method is to:
Clean out the cracks as best I can (don’t have a chisel so was using a screwdriver)
Apply PVA water solution
Mix and apply mortar (have tried very hard to get the water content right)
Previously kept wetting it so it didn’t dry out this time I covered it with plastic
Successful in one corner but the others keep cracking and collapsing.

Any ideas about what I’m doing wrong?
Thanks in advance
Sam

 

[Johnny Allround]

Have you got a picture of the whole window. The reason why I ask I came across this problem and ended up trimming all the reveals. Got rid of the problem and looked mint. Hope this helps.

 

[Sam6970]

Uploaded some more photos

 

[Johnny Allround]

I would have no hesitation in trimming both reveals with the same colour as your doors and side frames and a nice silicon line to finish it off perfect.

 

[lostinthelight]

Its a mistake to use pva ,if you use anything use sbr. I never cement or fill down on to the cill but leave a few mm gap which allows for any movement/ expansion which I caulk with ct1. I use sand and cement mixed with sbr or
Toupret Touprelith.F filler
This may or not be the correct way but has worked for me.