a) it needs to be on in that forum.
b) The pulldown menu needs to be on "Allow UBBCode" when the post is made.
If any of those are off, then it turns it off for the post. Beyond that "UBBCode" is pretty general - you'd probably need to produce specifics. There are a number of invalid things in posts which prevent the various UBBCode elements from converting.
Example - an image tag which doesn't contain a URL to a valid .gif, .jpg or .png etc... Various security checks in place. Quote marks inside of tags - etc..