Fillable form "tick" mark not showing up

Hi 

I need help desperately please. This issue happen only in some situation which i am still trying to determine the root cause.

1) I have a fillable form with checkboxes. 

2) I will fill up the form, put a signature in the the PDF. The form will be locked.

3) I will send this form to user. user supposed to sign using Adobe Reader and return the PDF to me

Now, this is the strange part.

4) User returns the form to me via email. when i open the form using my outlook client, the "ticks" in the checkboxes seem to disappear.

5) if i open the same mail using OWA, the "ticks" are there. 

May i know has anyone encounter the same issue as i do ? Is there some bug in outlook client that is causing the problem or perhaps some settings that the email server might have prevented the ticks from showing up ?

for some advice please.

thank you

Yiang