What Dilution ratio are you using ? Depending on severity of the dirt - dilutiuons need to be adjusted , it sounds in your case that trigger / pump spray may be a better option for you to help soften, remove in prewash format

 

Im following the recommendations from Labocosmetica who sent me this table.

 

For the cleaning of rims and tyres dilution 1:3 to 1:10; for the cleaning of the lower sides of the
vehicle, rails and bumpers or to remove gnats dilution 1:20; for the cleaning of very dirty vehicles
dilution 1:50 dispensing the product from the bottom upwards; for washing of medium-dirty vehicles
dilution 1:80, dispensing the product from the bottom upwards; for frequent washing dilution 1:100.
DISPENSING WITH THE FOAM GUN SYSTEM: in this case, it is advisable to respect the
normal dilutions, obtaining the output delivery as those recommended with the spray dispenser
described above. The final dilution varies according to the high-pressure water cleaner used.
We generally recommend diluting PRÌMUS 1:5 for winter dirt, 1:10 for summer dirt and for
weekly washing. After application wait a few minutes and rinse with high pressure.

 

I love labocosmetica products but I do find some of the instructions not very clear when you read it from the bottle. The little boxed off section they have on the bottle with the 1:5 and 1:10 is much clearer than the full on description that isnt all that clear when you first read it.

Discover PRÌMUS 2.0: The New Standard in pre-washing for cars and motorcycles in car detailing

 

From my experience it can’t be used as a standalone foam pre wash,it needs to be applied with a sprayer first on the lower parts of the car.

 

Strange, as I've found it to be one of the best pre-wash foams, as C&S have stated I use it at there ratios to great effect. I rate it up there with BH touch-less.

No pre-applying in a spray bottle first either, but that's just my experience of course

 

like adsk2 says I've also found it to be effective as a foam, I've got both touch-less and primus and rate them both highly.