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:
![](/legacyfs/online/430872_pastedImage_1.png)
I right-clicked the Point_relatedTimeData layer in the Contents pane > Sharing > Share As Web Layer.
In ArcGIS Online I see:
![](/legacyfs/online/430873_pastedImage_2.png)
![](/legacyfs/online/430874_pastedImage_3.png)
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!
![](/legacyfs/online/430875_pastedImage_4.png)
We see the table included in the web layer:
![](/legacyfs/online/430876_pastedImage_5.png)
And when it is added to an online map we see the related table functionality in place:
![](/legacyfs/online/430884_pastedImage_7.png)
Does that work for you?