Tile not sticking firmly to Haribacker

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Hi. I am having trouble with a tile project that I am doing. It is a floor with alternating 6x6 and 12x12 tiles. I completed the project and after a day the grout cracked before I sealed it. I noticed that the floor felt "spongy" when you walked on it but only on the larger tiles. I prepped the floor by taking off the old Lino down to the subfloor. I then put down 3x5 sheets of 1/4" Hardibacker and screwed it down in every recess. When I was finished, the floor was very solid- I did have to screw the sub floor in a few places b4 installing the Hardi. After that I cleaned the surface b4 starting with the mortar. I did use mortar that was premixed (not a smart move I know) but don't believe this to be the problem (I'll explain why in a few). I put down enough mortar that I could set tile in about 5-10 minutes. I used a wet saw to cut, but made sure the tile was towel dried b4 setting in place. There was no kiln dust on the tile, and I didn't allow anyone to walk on it for at least 24 hours. After a long weekend (3 days), I did the grouting as I normally do. The next day I went to seal it, and just from walking on it, the grout was cracking.
I waited about a week to see if the tile was still feeling spongy and noticed that they were still moving slightly (rocking about 1/4" in each direction corner to corner). I then removed about 25 sqft of the larger tile and a few of the smalls. I bought a maximum strength adhesive and reset the tile (after taking the same prep steps as stated b4). I am still feeling that the floor is spongy and can see the tiles I reset moving as they did prior. It has been a few days since the tile has been set and I noticed the extra mortar that was left in my mix bucket was not set up fully (it has been in the same room the whole time and this is why I dont think that its two bad batches of mortar) I live in Michigan and it's cold!! I cranked the heat up for a few days to see if it was the weather factor (the floors tend to get cold in this house because of bad windows). Still spongy. I now have put a dehumidifer in the room to see if it is a humidty factor. I have not regrouted and am in need of some help from someone with extensive tile experience. I have done several tile jobs in the past, but am a finish carp. by trade. The materials I used were the Rialto series tile from Lowe's and Laticrete mortar and grout. The maximum strengnth adhesive I used was from H.Depot by Omni good for tile upto 16x16 on the floor. I used a 1/4" notched trowel and 3/16" spacers. Please let me know what you think. Thanks