Are you saying it successfully sent the email 10 out 10 times but only included the attachment 4 out of 10 times? If the former, then I have no idea because I've never seen MAPIEx fail to attach. If the latter, then it's because MAPIEx is the worst and I'd switch to using any other solution (Outlook VBO, SMTP, Postal Service...).
Dave Morris, 3Ci at Southern Company