Rendering / plastering small garden wall... problem

Hi
I'm Rendering a small garden retaining wall...
And Having a bit of a problem with the mix
If anyone could help...

the wall is 2 and 3 solids high galvanized corner beads tops and corners...new blocks

Beads aren't sitting very proud so I need a very thin scratch coat and topcoat

I did a test today and The problem is the plastering sand seems to be very corse/gritty and won't work into the beads, tried it with and without a scratch coat, when I tried the scratch coat because the plastering sand was corse I couldn't get a fine enough coat..

Then tried adding a bit of mortar sand into the mix which helped a bit but still when floating off the plastering sand seemed to be falling away from the beads..

The mix just didn't seem muddy/ creamy enough
If that makes sense.... cement mixer was used

Mix was 4 kilsaran plastering sand
Cement 1
Waterproofer plasticizer and a bit of sbr

Any help would be appreciated
Thanks