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From your pictures it looks like your cauliflower may be experiencing a condition called "buttoning." Buttoning can occur in young plants beyond the juvenile stage if they are exposed to several days of cold temperatures. Delay planting until after the last frost. They should not be planted when temperatures are consistently below 50 degrees.

Weather is one of the most limiting factors in producing cole crops. Broccoli and cauliflower perform best with cool daytime temperatures (70°-85°F), lots of sun, and moist soil conditions. Cauliflower produces best in a fertile, moist, well-drained soil which includes both soils high in organic matter and sandy soils.

Buttoning can also be caused by insufficient water, a shortage of nitrogen, excessive salt, or weed competition.

Following are a few links to fact sheets with information about growing cauliflower:
http://extension.usu.edu/files/publications/factsheet/HG_Garden_2004-04.pdf
https://hortnews.extension.iastate.edu/1997/3-21-1997/broc.html
http://hortupdate.unl.edu/Broccoli


 

"They should not be planted when temperatures are consistently below 50°F" The minims or the maxims? I always transplanted the cauliflowers and the broccoli in the same day. I haven't got the same problem with the broccoli. In 2016 I transplanted the seedlings APR 28 (Cleveland average last spring frost ==> APR 30) and MAY 23 in 2015 (I thought "buttoning" happened because I transplanted them too late and the summer heat came before the heads matured). In 2016 I kept them under .5oz row cover till I made the photos (JUL 20). Regarding the water I was on the too much side (because of a lot of rain in May and June). In 2015 a lot of the Brassica roots rot, last year only a couple before I took off the mulch. In the fall of 2015 I made a full soil analysis at the University of Massachusetts and they found that I have too much nutrients and recommended to add no more compost for at least a year. I may attach the result if you tell me how to do. The mulch kept the weeds at bay and after I took it I pulled out promptly the few weeds that came. So it is still confusing for me. Thank you. Mihai Gata

Your April planting may have been too early. Cool temps following transplanting is a major cause of buttoning. Also, older transplants are more susceptible. Following is link to a fact sheet from Michigan State University which actually recommends a transplant date around May 25. Transplanting too early, late April-early May, is more of a problem than transplanting later.
http://archive.lib.msu.edu/DMC/Ag.%20Ext.%202007-Chelsie/PDF/e1591-1982.pdf

This is an excellent material. I think I don't need more information for now. Fall production cauliflower is much easier to grow. Unrelated to cauliflower I wonder why the question is treated as private because I checked the box to be public. I don't see any reason other people not to benefit of the info here. Thank you very much. Mihai Gata