在其中fortune名詞詞條下的解釋是：
for?6?1tune /ftun; NAmE frt/ noun
1. [U] chance or luck, especially in the way it affects people’s lives: I have had the good fortune to work with some brilliant directors. By a stroke of fortune he found work almost immediately. Fortune smiled on me (= I had good luck). note at luck
2. [C] a large amount of money: He made a fortune in real estate. She inherited a share of the family fortune. A car like that costs a small fortune. You don’t have to spend a fortune to give your family tasty, healthy meals. She is hoping her US debut will be the first step on the road to fame and fortune. That ring must be worth a fortune.
3. [C, usually pl., U] the good and bad things that happen to a person, family, country, etc.: the changing fortunes of the film industry the fortunes of war a reversal of fortune(s)
4. [C] a person’s fate or future: She can tell your fortune by looking at the lines on your hand. see hostage, seek—see also soldier of fortune
這樣，根據意思的話，try one's fortune中的fortune只可能是1和4的解釋
看1啊，不可數指運氣或機會，尤指影響人們生活的方式，意思呢約等於LUCK，按我理解有種泛指的意思。
4 可數指個人的運氣、命運或前途，（妳可能要講，用FATE不是命運的意思麽，其實它也有運氣的意思，這個用法不比用作命運少，牛津雙解裏還有destiny這個解釋，可能改版的時候覺得和fate的意思重復，刪了，destiny也是命運和運氣的意思）
try one's fortune它在英語中不是個原始的固定搭配，是後來的壹個搭配形式，在這個句子中有特指的意思吧，我覺著釋義4就是特指，所以它就是可數了。
不過還有個句子They are going to try their fortune once more.他們想再碰壹次運氣，這裏就不可數，這個用的就是1的釋義吧。
O(∩_∩)O~當然這是我個人之見啊，妳琢磨看看哦