Normally Process Update shouldn't be hit & miss.
I stumbled across this blog post on MSDN:
Different Kinds of SSAS Processing in simple words…
This particular line is interesting:
ProcessUpdate is inherently slower than ProcessFull since it is doing additional work to apply the changes.
This means there is no actual reason to do Process Update instead of Process Full, so I would just stick with the last one.
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MCSE Business Intelligence - Microsoft Data Platform MVP