Ajax edit of substitution fields in Custom HTML fields will only work with [cb:userfield field="FIELD_NAME" formatting="div" /] or [cb:userfield field="FIELD_NAME" formatting="span" /] substitution usages. I've tested and confirmed this as working fine, but it limits how it can be displayed as it'll output the full DIV display of a field. The formatting in the HTML substitution is important as it needs a container element for the ajax to function properly.

You need to set the two fields your substituting in to be ajax editable and not the Custom HTML field. Also ensure the field name you've supplied is as shown in CB > Field Management. I don't know what more to advise as it works perfectly fine in all my tests. You may have a conflict issue as per your other topic regarding CB Conditional.