Even when she has come into heat (yay!) and you've gotten the AI guy out in time (double yay!), she may not "take" the first time.
In the harder winter months, these problems often mean she'll be far less likely to come into heat, her optimal breeding window within that heat will be shorter, or she may be less likely to take to breeding.
If she isn't getting the nutrients she needs, you can expect dramatic weight loss, especially in cold weather, an irregular heat cycle, and inability to take to pregnancy if she does come into heat. Remember, she'll need to get enough nutrients for her own maintenance, to make the milk she's giving you, to build a nice healthy calf, and don't forget--she's got to have enough energy to birth that calf without problems.
These notions are based on the common myth that a lot of unbred does and doe fawns come into heat during a one-week period in mid-December -- the second rut.
A limited number of does may come into heat early, but the rut peaks in mid-November, and that peak-rut period could be considered analogous to the predictable heat of August.
In the past some writers have promoted the idea that a few older does will come into heat as early as late October.