Do you have an "in map" relate or a relationship class? Are the feature class and table stored in the same geodatabase?
I just have an "in map" relate (not a relationship class) here:
I right-clicked the Point_relatedTimeData layer in the Contents pane > Sharing > Share As Web Layer.
In ArcGIS Online I see:
So, though the table is published, it isn't actually related.
Next try, actually create a relationship class between the feature class and table, then share the layer.
There we go!
We see the table included in the web layer:
And when it is added to an online map we see the related table functionality in place:
Does that work for you?