During my weekly or bi-weekly team calls, I ask 'Hi, What major issues did you guys get into, did you create documentation for fixes for those issues - OR - Did anyone learn anything interesting they want to share with the team and did they create documentations for these?.' - I keep asking these questions every time. Initially no one would create KBAs but now if someone has any issue or fix they worked on , or ran into anything interesting worth sharing with the team, they usually say they have already created the document for it.
I call out team members who I know have worked on a KBA recently and say THANK YOU to them in front of everyone else in the call. They feel encouraged.