Reasons for negative iodoform test
As @Waylander pointed out, it appears this reaction has not been performed and/or recorded in any literature, so it is quite dangerous to speculate. 

But keeping that aside, A 3D perspective reveals that abstraction of protons from the methyl group in quite unhindered.



Hence, the triiodo intermediate is well anticipated.


However, a quick glance at spatial orientation of iodine atoms reveals the reaction may be dead slow in the next step.




Notice that the Burgi-Dunitz trajectory, which we may assume the incoming nucleophile to take, is hindered by the large iodine atoms and the methyl group.

It is quite safe to assume that the attack at the carbonyl carbon is unfavoured, preventing the release of the $\ce{CI3-}$, and ultimately $\ce{CHI3}$ never appears.

EDIT: Apparently there is some relevant literature available for similar compounds, as mentioned in this answer. Thanks to Mathew for searching and pointing it out.