Hi
I have come across this issue and below where few of the suituation :
--> when the of respective virtual machine is high and the quiescing operation is unable to push all the data to disk, while further I/O is created.
-->If you try to create a quiesced snapshot on a virtual machine that does not have free space. That is, if the free space on the virtual machine is Zero or Nill .
Try to check if reboot does change anything for you .
Please consider marking this answer as "correct" or "helpful" if you think your questions have been answered.
regards
Gaytahri