I'm going to assume that you don't have a bunch of cows. I'm going to assume that if you have to sell this calf at the sale barn, you don't have a bunch of money you're wondering what to do with to replace her. I'm going to assume that you're somewhat attached to this heifer.
Based on all those assumptions, it's my opinion, and obviously everyone has one, that you should explore all options before selling her. Sometimes they just won't breed. AND sometimes there is something that can be corrected, like all the reasons stated above. Another common problem i didn't see mentioned is that these heifers don't drop the egg at the right time. They come in heat you breed 'em, the semen dies and then they drop the egg. A simple CHEAP injection will fix that right up. Sometimes problems can be solved by using her as a recip and getting her pregnant with an embryo.
As to some of the other comments, like.........."she'll always be hard to breed." Not necessarily true. If its a hormone type problem, having a baby can get all that fixed. If it's that timing problem, it may never happen again. If it's a physical problem, like something wrong or a blockage in her cervix, maybe not so much.
And as to the comment "would you want replacements out of her?" Low fertility is NOT a highly heritable trait. IF it were, there would be no infertile cattle. They would have long ago died off from the inability of those lines to breed back. The response to the drugs used in embryo transplant, which is sometimes viewed as "high fertility", does seem to be highly heritable. But the inability to breed is not HIGHLY heritable or there would be no problem animals today.
At some time you will come up to a point of diminishing returns, where it costs more to pay someone to try to find her problem than it would to replace her. That might be a good time to let her go. BUT.......I'm assuming the cattle business is not what you depend on to pay for your meals and shelter. Therefore, you DO NOT have to treat this a a business. If this heifer means something to you, I would explore all the possibilities before giving up.