I never saw that behavior before.
Can you upload your file or a file that only contains the check boxes somewhere and post the link here?
you sample looks fine and I don't have any problem on my system.
All boxes show the mark as expected.
I found a description in the XFA specs about the marks, which let me think that it is your Reader that causes the effect.
Thanks for the link. The check mark appears on most of the systems but appears as in the screenshot in Windows XP Embedded machine with Adobe Reader 9.5. Also any idea what is the ASCII for the check mark in adobe forms? Also is there any relation to font on this?