I believe your problem is caused by manufacturing variations in the Wixels. The Wixels you have are from three manufacturing periods, in which we changed crystals. I'll call them phases to make them easier to talk about:
Phase 2 Wixels (March 2012 to September 2013):
Phase 3 Wixels (September 2013 to October 2014):
Phase 4 Wixels (October 2014 to present):
The clocks on phase 2 Wixels tended to run a little fast, and this was improved in phase 3 and improved more in phase 4. However, phase 4 clocks tend to be a few ppm lower than nominal, so the frequency difference is most pronounced between phase 2 and phase 4 Wixels. What we have seen is that while phase 2 Wixels work with phase 3 Wixels, and phase 3 Wixels work with phase 4 Wixels, phase 4 Wixels have trouble communicating with phase 2 Wixels.
You didn't say which Wixel you are using as the receiver, but I suspect it is one of the phase 2 Wixels because every phase 4 Wixel is performing poorly for you. Is it feasible for you to try using one of the phase 3 Wixels as the receiver? Please let us know if that resolves your issue. If that does not work for you, we can also try changing the tuning in firmware.