Just get an Ultra Pro because it comes with 1 piece bars, reinforced fork, and a 1 piece deck and for only $99, that's a good deal, I have one and I love it! :)
I know your problem, nothing is wrong! I am not trying to be a jerk, but mine does the same thing and after about 30 minutes I just use my hand to tighten it again. Nothing is wrong with Ultra Pro's it's just how they are. :)