No rating increase when winning a rated game ?

It's because people use maia1 to farm

After a certain while of playing against maia games automatically become unrated even though you still see rated. This is to prevent possible attempts of rating manipulation due to maia repeating the same lines over and over.

#2 not sure what that means ?

What Deadban said is not quite accurate but the general idea is true. When playing against bots, games may be force-unrated if Lichess thinks you might be farming rating, for example by continuously playing the same moves which you found out the bot doesn't play well against. Some bots always play the same moves and therefore will fall into the same traps over and over.

If you want to gain rating, just play against humans.

@WillM1966 never heard of the word "farming"? Anyway, #3 and #5 explain it well enough?

Here's an extreme example. Back in the 1980's the Chess Federation of Canada used an older rating system where "participation points" were included in the calculation, based on the idea that everyone gets better as they play, so the rating pool should not be quite so zero sum.

So these 1500 players from Timmins, Ontario, decided they weren't highly enough rated. So they submitted a whole bunch of crosstables of tournaments that they conducted amongst themselves, and the net result was that they all went up to like 2400, and I think they wanted to qualify for the Canadian Championship or something.

Their ratings were adjusted. But that's one example of farming.

Interesting story, thanks for sharing!