Sodium Alginate forms weaker bonds (gel) in high acid foods - eventually it won't gel at all if the ph gets low enough.
Raspberries are quite low ph ... here is a sheet with common food stuff and PH levels.
Raspberries are also fairly low in calcium and that which is present is likely bound up in pectin (not available)
Gere us a sheet with common calcium levels in food. It doesn't take into account availability though - but it is better than a guess.
In short you're going to need to use a touch more sodium alginate in the water bath (and maybe even use a gelling agent more suited to lower ph levels) Most preparations I've read are 2.5g to 500g water ratio (0.005) your ratio is (0.004) not a lot but maybe enough when taken into account with all the other variables.
You will also have to use way more calcium lactate.
A common ratio for acidic foods is 0.02 yours is 0.005
I would start at 0.015 and work upwards.
For your recipe this would be starting at around 15g per 1000g and working up to 20g per 1000g of coulis.
Once you hit 0.02 ratio your likely going to find better results by adding xanthum gum rather than increasing calcium lactate even more.
I'd start with 1g per 1000g (ensure your hydrate the xanthum gum very well)
Hope this helps.
Also what size are you trying to achieve? sometimes the larger ones require more alginate to set faster than they break up when transfering to the bath.