I used to run into this often. I even had times when going into the properties of the table wouldn't work. What I do now, which always works is the following;
In normal mode, place your cursor in the first cell of the table,
Go to HTML mode, the cursor should still be at the begining of the table, and you should see the following above, but close to your cursor:
<table border="0" cellpadding="4" cellspacing="3" width="592">
Add align="center" right after the word table, so it looks like this:
<table align="center" border="0" cellpadding="4" cellspacing="3" width="592">
Linda Adams helped me with another problem I had with aligning text and graphics, in which tables are used. That solved a number of my problems, and I am using tables a lot more now than ever with great results. The centering issue comes up almost every time, and adding the align command always works.
Jon Holmen