DT,

Given that the process conditions are the same in these days, I would
first check the XeF2 source
to see whether etchant supply is consistant. Maybe your crystal was
pumped away?

Humidity or poor dehydration can also reduce and eventually stop etching.
If you look at the etched
 surface and find white deposite, which can not be removed easily, it's
probably the by-product
from XeF2, Si and H2O. It is very important to bake your sample well
before etching.

There is a very good paper from Professor Pister about XeF2 etch process.
I do not have it with me
now. You can email me at [email protected] if you can not find it.

Best,
ShuTing