Hmm that is really strange. Make sure you have Payment Action set to "complete sale" in PayPal Website Payment Standard Settings from Admin > Store Configuration > Payment Settings > Payment Methods.

Also from your Paypal Account, try going to Profile > Website Payment Preferences and turn on "Auto Return" and set the return url to yourubercartsite.com/cart/checkout/complete.

That failing, these guys had a slightly different problem, but some of the info might help:

http://www.ubercart.org/forum/support/17260/ubercart_not_receiving_notif...

Good luck!

James