You have to understand that many streaming channels, like The Roku Channel, have to lease the streaming rights for most of their programming from the program owners.
Available leases assign streaming rights for certain geographical regions over specifed date ranges. It's quite common for leases only to cover some seasons of multi-season programs, often with the latest seasons of long running programs being split out into higher priced leases.
Some leases may be obtained by several different channels at once, but others may leased exclusively to only one or a few channels and are unavailable to others until the exclusive leases expire.
As a result of all this, programs and seasons of programs, bounce around from channels to channels all the time as leases expire and new ones are obtained.
It could well be that the last 3 seasons of Doc Martin are currently leased to channels requiring scriptions or per episode payments.
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