No Gedcom MARR tag for a marriage will be produced unless either the Family Relation is set to indicate a marriage or you create a Union for the Family. Without one or the other then only a FAM tag is generated.
Note that the horizontal line on a GenoMap joining two individuals does not in itself indicate a marriage, merely a family.
It is impossible to examine you particular case unless you send an example. If you are prepared to send me the .gno file then use the Send File dialog (http://www.genopro.com/help/send-file/) on GenoPro's File Menu when you have your example open in GenoPro. Just Send to my Username (Ron).
Tried deleting the marriage and adding it back in but no luck
Could someone have a look if possible - when the gedcom is produced, the marriage isn't there?
Thank you
Dave