ive put some windows into a regular brick built building, But when i have pointed the window horns the pointing has cracked where is meets the original brickwork once dried. i fitted a few windows over a period of time so have experimented a bit with different ratios like 3 parts sand to 1 part cement and then 4 parts sand to 1 part cement.  I also tried wetting the bricks before trowling the mortar in but it still cracked.

 

this isn't my image but it shows where the door sill horns extend into the brickwork and then the excess gap is then filled with mortar. plus you can see where this one has cracked around the edges like mine all have.

 

So what am i doing wrong? is pretty frustrating.

 

the muck will always crack as the plastic cill moves a lot especially in the sun.

the ones i did yesterday have started to crack already.

 add some grano dust to the mix it will still crack where it meets plastic though you have to silicone it there.

i did silicone that bit but its cracking where it joins the wall,

Put a bit of dpc on the sill before filling the gap with mortar, and that forms 
a slip layer for when they expand and contract. No more cracking.

Sand & cement shrinks as it sets. I’d suggest a weaker mix (1:5 cement:sand) and to put the material in as dry as is possible (but obviously still mixed properly.) Dry the muck out on a piece of timber whilst prepping if need be. 
If possible,inspect the work the day after and touch in where needed. 

some good points raised. i will try and mix it weaker and dryer, ive not got any DPC so will try and scrounge some.