Low light intensity may be the problem:
[snip]
"Generally, the rate of photosynthesis relates to light intensity, but not proportionally. The importance of light becomes obvious in the winter, when it is in short supply. In the short, dull days of late fall, winter, and early spring, the low daily levels of radiant energy result in low levels of carbohydrate production. Not only do the poor light conditions limit photosynthetic productivity, but also the limited carbohydrates produced during the day are largely expended by the respiring plant during the long night. The low supply of carbohydrates available in the plant during the winter seriously limits productivity, as evidenced by the profusion of aborted fruit." http://bit.ly/1aHyfT6 [p. 13]
the cucumbers are in a greenhouse about 10 years ago.