I used sand mix (with acrylic fortifier) for the pre-slope I built for a new shower pan. After the mortar sets up(two days later) I checked the surface by knocking on it with a stick, I heard hollow ring instead of solid thud in some spots. What does this mean? The material in the spots where I heard the hollow ring was very low in strength and very easy to break. Should I remove the material from these spots? The fact that a PVC water proof membrane and more sand mix mortar plus stone tiles will go over this surface, does it matter that much that this pre-slope has some weak spots in it?

This pre-slope is built with wire mash on a poured concrete floor. The thickest part is 1.5" and the thinnest part next to the drain is about 1/4".

Sounds like there was too much air in the mix. When you put down the mix, did you trowel it immediately or did you "work it in" place? If your going over it w/ a membrane and another mortar bed, I guess filling in the weak spots should be OK.
For proper tiling procedures you can go to www.tileusa.com This is the Tile Council of America web site. They'll send you a free ceramic tile installation guide.

Thank you for the response. The mortar was hand-mixed to the optimum dryness to the best of my ability, it balled up when squeezed by hand and yet still dry to the touch. I packed the mortar in with a wooden float. I must not have packed all of the material in evenly to have some air left in it. I removed some material from the spots with low strength. Can I use the regular cementitious thinset to fill in the void I created? The void are about 3/8" to 1/2" deep and 10" x 10" in area.

If your going to use thinnset to fill these areas. Fill the voids and fiberglass cement board mesh tape the seams, then skim coat over the surface w/ more thinset.
Good luck.

Would the sand mix mortar be better? I thank you.