So when you long press on the email and choose delete, does it bring up a confirmation box and if so, what does it do when you hit delete? Nothing? You might hit the menu key and select Refresh. Another thing you could try is go to Settings>Accounts & sync settings, then select your account down at the bottom. On the following page, verify that the Sync Gmail box is checked for starters. Whether it is or it isn't, make sure it's unchecked, then check it; this will start a manual sync, and may correct the issue.