flymo garden blower-vac cutte headr keeps breaking

Last week bought Flymo Turbo 2200 garden blower-vacuum for collecting bags and bags of leaves from adjacenet L.A. public park (we have no trees!)
First day, the cutting head broke in two after less than a hour's use. Focus replaced the machine without question.
Went out again today with new machine. Aftyer about an hour, new machinie's cutting head spit in to.
Bpth happened after replacing emptied bag, but both times (as I'm new to this machine) I followed the maker's instructions, and both times the bag assembly clicked audibly into place.
Anyone had similar problem, or suggest why this has happened twice in two short uses of the machine?
(tried garden newsgroups without success, hence posting here. About to contact Flymo, but similar experiences from other useers might help).
Thanks for any suggestions/comments.
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