I had a Fraxel 1550 only, not the 1927, and after more than 3 weeks I'm still very red, why?

I don't know your skin type but the settings are very aggressive. If technique is not correct, it is possible to cause bulk heating and burn the skin. I can't say exactly what the problem is without pictures or performing the procedure. However, redness persisting this long needs therapy to prevent PIH. If type 3 or lower, a VBeam or Excel V laser would be needed. I'm not sure what your skin care is but that could be contributing as well. Perhaps preventative Retin A and hydroquinone are needed.

Please follow up with your treating physician.  You are likely still healing from your fraxel treatments and likely are still pink due a very strong treatment based on those setting. If you are still concerned I would speak to your treating physician.  Good luck with your recovery.