fluctuating temps and irregular watering are often the cause.
try a fresh batch and keep well watered.

How long have they been in the ground? If they are left from last year, they may bolt. Beets are a cool weather crop, so warmer temps is another reason. Also what Sb said.

In the UK beetroot is grown is sown in spring and harvested in autumn Bathgate, probably due to our climate.

OK interesting. I need to have an early spring crop, then I can plant another crop in late august, which would be ready in time for Thanksgiving. Our hot dry summers will cause beets to bolt

They may have the same problem in the south of England but northern England and Scotland aren't hot enough to cause the beets to bolt, and this year the norther part of the country has been cold and wet. Think SBG hit the nail on the head.

All very interesting thanks very much to all