Ceramic Tile Cracking
I think your sub-floor is 'bouncing'. You didn't say what ALL the layers are under your tile are, nor did you say what your joist-width AND spans are either. These variable come into play and help determine if the simple install of just WOnderboard is enough. Perhaps you needed an additional layer of 1/2" plywood? (Who knows.)
The bottom line is: You could fix the problem but it will most likely return. You could look into what could be done to reinforce the existing floor (from underneath, but I'd do that BEFORE I'd fix the floor.) Of course, the last thing you want to do is have the new floor ripped out. BUT, that's your call (assuming it's needed as part of the 'permanent fix' ...)
My best to ya and hope this helps.
Jay J -MOderator
At this point, it might be your best option to prevent further cracking. You might have to do some regrouting, but maybe it'll last.