How do I get new tile to stick on an old slightly uneven floor?

What did you use for backer board? Typically you would pour a thin layer of self leveling concrete designed just for this purpose, to level the floor before laying down your flooring.

If you can’t level the floor you will have trouble using ceramic or porcelain tiles.because of the uneven areas will have gaps and not stick. Have you finished laying the tile? In the places where the tile is not sticking you may have to put a leveling compound down. This may cause the tiles to be higher then the rest of the floor. Sorry no really good answer when it comes to uneven not level floors. For a good tile project the floor must be level and in older homes that can be disappointing. There is a product that has a little give in these instances it’s called Snap Stone http://www.snapstone.com/ this can be found al local DIY stores. It is actually a floating tile floor that snaps together and has some give in less than level areas. Hope this helps.l

I have a tendency to learn the hard way too, from experience! You will know next time though