Please check your log (under Seamless Donations > Log in WordPress Admin).
If it shows Pending - then your PayPal account is not approving the payments. Make sure you:
1) have verified your PayPal account - check and confirm that your PayPal account has a verified email address. If the address in your PayPal account is not verified, payments will not automatically be marked as complete.
2) do NOT have Payment Review enabled on your PayPal account
3) have set up the currency in PayPal to match the currency you have set up Seamless Donations to receive in
4) if the donor elected eCheck - those can take several days to clear - and until that happens it will show pending
Hope this helps...
Let me know if the logs show something else is amiss.