Can't sign in:
A successful sign-in will show the "Welcome back" Game Center banner and your player name will appear at the bottom of the lobby. If this doesn't happen, you need to verify that your Game Center account is set up. Open the Settings app, tap Game Center, and verify that a valid Apple ID is being used and that a Game Center profile exists.

Invites don't work:
Make sure that neither you nor your opponent hasn't disabled Game Center invites. Open the Settings app, tap Game Center, and enable "Allow Invites" and also, if necessary, "Nearby Players". If using iOS 10, iCloud should be active (if not, open their Settings app and tap "iCloud").

On iOS 10: invitations may appear in the Messages app. If you have notifications blocked for Messages, you need to manually check to see if any invites have arrived there. When sending invites, choose contacts that you know have an active Game Center account.

Taking a turn doesn't notify the other player:
This may happen sometimes. The only workaround is to manually get the match update by opening the lobby and maybe also refreshing the match list (pull down and release).

Rules didn't show for a match invite:
If you tap an invite's notification instead of its panel in the lobby, the invitation will be implicitly accepted. You can still see the match's rules by choosing Rules in the match screen, but to decline the match, you or your opponent must resign.

The points for a play are lower than expected:
Be mindful of score deductions. Depending on the match rules, loosened tiles which are moved incur a cost equal to their value, multiplied by any letter bonus. When changing or replacing a word covering any bonus, any unmoved loose tiles having the same letter as the originally played tile do not increase the score. As a rule, treat all such tiles as being unloosened before the turn score is even determined.

No sound effects:
Tap Settings on the lobby menu, and switch Audio on.

Player stats command disabled:
Stats are unavailable until you've played at least one match.

In the lobby, a match panel doesn't show the opponent's identity:
Sometimes when starting a match, Game Center cannot provide the opponent's name or icon until he/she takes their turn. Continue playing, and their identity should show up.

Match panel appears orange in the lobby:
The match exists, but may not be openable. If the match data is unreadable, it's probably because the app has been updated and uses a newer data protocol (or your opponent has updated their app and has invited you to play). Before playing, verify that you and your opponent have updated to the latest app version.