I have created a hollow model that has 1.5mm walls. 

I am now attempting to drill drain holes in the model. It gives the error:  "Fail: Not hollowed out or too deep to dig" However, it is NOT too deep to dig and it IS hollowed out. It takes about 20 tries on the first hole to get this to work, and then about 40 tries to get the second hole to work. After that, I can't dig a third hole in the model. It continuously gives the fail message. I'm beyond frustrated and perplexed. I don't have any idea how to make this work properly. Any advice would be greatly appreciated.