OK, they turn Off completely, and they stay Off.
That sounds like a loose power connection in each of them. That is why I asked about, "Have they been gotten wet?" and "Have they been subjected to vibration or shocks?" Such an effect might loosen up, or (if gotten wet or damp) corrode the power connections.
Since they are about 2 years old, they are no longer under warranty. I advise that you have them opened, and the power connections re-seated. Then when closed up, the connections may be more secure, and may hopefully not come loose.
This of course, should be accompanied by a close inspection for any visible abnormalities and/or corrosion, in case something of the sort needs fixing.
Let me know if this answers your question - or, if you would like further instructions. Thanks.