How wide are the gaps between the slabs?

Sounds like you are doing everything right with regard to the mix. You could try using SBR mixed into the mortar, 1:1 with water. Mix it so that you hold it in your hand and squeezing it makes it hold its shape but no water comes out. The only other thought I have is that you need to rake out the joints to ideally 30mm or more. Use a stick if you don’t have the proper tool to really compress the mortar then don’t let it dry out too quickly. If you point late in the day you could cover it with something to make it sweat or use a fine watering can to wet the mortar without making it run and mark the surface. Once it’s gone surface hard give it another watering to slow the drying process.
Another thing to try would be 2 sharp to one soft, or all sharp, it does depend on the quality of the sharp sand, sometimes it is full of relatively large bits which make pointing difficult but I normally use just sharp sand if I can.
There doesn’t seem to be a consensus on pointing paving everyone seems to have their own mix, I’ve seen some recommending plasterers sand but I’ve never tried that. One thing I did try was polymeric sand for jointing paving, I found it useless, weeds were coming through it within the year.
Best of luck you may need to do a bit of trial and error.