Refused an exchange of merchandise (wrong size)

I presented a pair of shorts for an exchange for a different size and was refused.  I was told that I had a check on file and that I could not exchange the merchandise.  I have a Wal-Mart card and have not used a check in years at any Wal-Mart.  I make purchases regularly at the same store.  I called the 800 number provided and I was told they had nothing on file against me.  Then I was informed to call an 877 number and they have yet to return my call although the message says they check their messages daily.  How do I get this resolved?