Hi,
Please ensure that thermal plane net name and via net name both are same.
If it is same then i think that the spoke width you have entered in the plane class and parameters tab is so high than the via pad size.
Try with buried option in the plane class and parameters tab. See the attachment.
I think this one will help you.
If it is not working please give some snaps to understand your problem better.
Regards,
Sajiv