Unless you are fast don't spread too much mortar at a time.

Mapei instructions:
Place the tiles firmly into the wet mortar. Push the tiles
back and forth in a direction perpendicular to trowel
lines, to collapse the mortar ridges and achieve
maximum coverage. Ensure proper contact between
mortar, tile and substrate by periodically lifting
a few tiles to check for acceptable coverage.
Remove excess mortar from the joint areas so that at
least 2/3 of the tile depth is available for grouting.

Could be the mortar is a bit too dry. They say it should be mixed to the consistency of peanut butter, but I would amend that to "creamy peanut butter", if you know what I mean. Also, after waiting 5, mix again, and also remix the bucket periodically (a couple quick turns of the mix) with your margin trowel (rectangular hand twowel).

And, as mentioned, you have to "set" the tiles into the mortar. I whap them "more than gently" with a rubber grout float, but a little bit of the push and wiggle can do it also.

Full disclosure: I'm not a tile pro, but have done more than a handful of these jobs. One has to do to that to gain the hands-on experience. Final caveat: don't follow the methods on the diy tv shows, especially the one that rhymes with "cr@p-tastic". They are mainly good for design ideas and that's about it.

Good luck with the project.

BTW, don't make the mistake of adding "a little bit of water" to a mix that's drying out too quickly. That means you mixed too dry to begin with - dump it and make a fresh batch.

Realize that when you use a 1/4" notched trowel, after the tile is set and in embedded in the thinset, the 1/4" square ridges of thinset should be compressed to a nice even 1/8" thick layer of thinset that fully adheres the tile to the backer board.

Question...or maybe a comment...after you let the thinset slake for the 5 minutes, make sure you mix it up again prior to spreading.

Make sure the backer board is clean. No dust.

You can try using a semi-wet sponge to wipe the cement board down prior to thinsetting. Wetting the cement board will prevent the porous and dry cement board from sucking the moisture out of the thinset prior to the thinset having enough time for a nice cure.

Try burning the thinset in first. This means using the flat side of the trowel to apply a skim coat on the backer board, you're trying to force the thinset into the pores of the backer board. "Apply a skim coat" really isn't correct, as you're really just filling any micro-voids in the surface of the panel. You're not trying to leave a measureable thickness. After burning in, flip the trowel and use the notched side to apply a measured amount.

Try burning thinset into the back of the tile too. Again, you're not looking to apply a thickness, think of it more like "wetting" the tile with thinset.

When you set the tile, press it in to collapse the ridges. Slide a trowel under the edge and pry the tile off. You should have near full coverage on the back of the tile, and there should be no umcompressed ridges of thinset on the backer board.

What size tiles are you setting? Ceramic? Stone? What size?

What kind of "cement board"?

It's Durock cement board. The tiles are 4.25" standard square ceramic tiles...seem to be the cheapest of the Majestic line. The bond with the Durock is fine. When the tiles do fall off there is no mortar on the tile, but the mortar on the cement board is really stuck on there. It sounds like I'm just having some technique issues which basically means that tiling is a PITA! I was hoping that I was just using the wrong adhesive.