Originally Posted by
ptle
It's most likely a very slow leak. I'd take the tube out, pump it up, and slowly inspect it underwater. Another way is to spray soapy water on it because bubbles will form.
+1
i had one of these. chances are, there's a very tiny piece of wire lodged somewhere inside the tire. get a cotton ball and run it around the tire. when it gets dragged, you'll find the sucker