Every time I post a question to the main site, it always seems there is something that I missed when posting it. How do I catch these issues?
Use the sandbox
Even for the most experienced challenges writers, catching all open questions and potential issues of a challenge can be difficult. That's what the Sandbox for Proposed Challenges is for: get help from the community to catch these things before going live on the main site.
As a rule of thumb, I saw that you used the sandbox once before, but sandboxing your challenge for less than two hours isn't terribly effective, as almost nobody will have seen it by then. I suggest waiting at least until your proposal has dropped of the first page (sorted by activity) or until a week has passed, whichever happens earlier.
Write more questions
As it is with many things the best way to get better is to practice, and question writing is no exceptions. Every time you make an error or a misstep you learn to avoid it.
There is a lot that other users can do to help you, in the sandbox, in chat and through the FAQs, but there is also a great value in getting the feel of a fun clear question. Forming your own intuitions and systems for writing and proof reading questions can be extremely valuable.
Read more questions
It's practically a cliche at this point, but they say that to become a better writer you ought to read more. And I think this is true of questions as well. Reading more questions will also help you build the same skills as writing questions.
This works especially well if you spend time to think about how the question works. Here is my checklist for reading challenges
Try the challenge. Even if just a bit try the challenge to see how it feels when you are solving it.
Form an opinion. Do you like the challenge? What parts do you like what parts do you think are bad?
Ask yourself "How would I write this challenge?" You don't have to write up a whole draft but thinking about this can help you to get practice at writing challenges.
I would agree with all of the sandbox suggestions and would add to leave your challenge there until there is at least some feedback. May be 2 days or may be a month. There is no rush to put the challenge live and waiting and getting it right can save a lot of downvotes. There is no rule that says we have to go to the sandbox and give feedback but many of the more experienced members have helped me there with previous challenges. I thank them for that. I have also written (not particularly golfed) reference implementations for all of mine. This helps me understand any ambiguities in what I am asking. Works for me and hopefully will help you too.