What looks like a people problem is often a situation problem.
I prefer this version, partly because it uses the word often rather than always and so it follows Bill Bailey's Law. But mostly I prefer it because saying "it's always a people problem" feels demotivating because we are all people and you get drawn into thinking you're the problem. As Deming made clear, the root cause of most problems is not the people but the system. But the word "situation" is simpler and somehow easier to grasp than "system". It's clear your always in the situation that you're in.
A related quote from Switch is
to change someone's behavior you've got to change their situation.
I like this a lot and it reminds me of the various quotes about acting your way into thinking differently being easier than thinking your way into acting differently...
Sometimes it’s easier to act your self into a new way of thinking, than it is to think your self into a new way of acting. (Jo Berry)
You cannot change how someone thinks, but you can give them a tool, the use of which leads them to think differently. (R. Buckminster Fuller - quoted in The Fifth Discpline by Peter Senge)
Adults are much more likely to act their way into a new way of thinking than to think their way into a new way of acting. (Richard Pascale's in Surfing the Edge of Chaos)
Or, to say it in reverse
Knowledge does not change behavior.
So instead of saying "it's always a people problem" perhaps it is better to say "it's rarely a people problem"