Honestly, I doubt that there's something wrong with the forum, table-wise. At most, some thing may be filtered that causes your table to break, which you would find out by viewing the source. I might be wrong, but I'm pretty confident on this one.

As for spaces between tables, bear in mind that any newlines will result in a new line being made. If you do not want any space between your tables, do not use new lines in your html. That's one of the most common problems people run into, and my guess is that that is your problem.