I am having the exact same issue when helping Uncle Cheese with the DatePickerField.
Bumping this since this is preventing me (and others I guess) from creating our own fields and using them in the CMS.
Like I said in the previous post, the requirements of my fields only get loaded if I reload the page they are used on, not if the page is loaded via AJAX. I have checked the other field types that SilverStripe uses and they don't have any other special code as far as I can see. They all use Requirements:: and they get their stuff loaded correctly.
Could one of the developers (or if someone else knows) please tell us the secret?