As the others have said, you obviously have voids below the slabs. I think Tony's right in that they're sitting on "dabs". With those voids, the bearing for the dabs is going to weaken.
Since this is a bit unusual, I'm going to stir things up by suggesting something slightly different. I would try filling the voids with slurry instead of dry mix. Ordinary 4 or 5 to 1 sand/cement mix ................... but VERY wet. Wet enough to be able to pour it into the joints. Use a plastic kitchen measuring jug, or a funnel.
As you can guess, slurry is self-levelling, and is likely to find its way around the dabs, and make the whole thing pretty solid.
Pour to the underside of the slabs......... then finish with conventional pointing.......
Or, fill to the top, let it semi-dry, then iron and brush the joints to finish.
If you're not confident about pointing......... you could rent a pointing gun (like a silicone mastic gun, only bigger.)