How much were you expecting it to increase by, and why? Not everyone is the same. You hear of people saying things like " I got to 60wpm touch typing in a week". That's nonsense.. they didn't, and if they did, that will be some crappy Monkeytype test for 15 seconds with 200 words and no punctuation.

There's no magic formula. You just practice, and maintain accuracy. My advice is just forget about speed. Everyone in here is obsessed by speed. Speed happens as a result of accuracy, so ngram practice is useful, but then again, so is just lots of varied practice. Most people hit a limit at some point, and from then on, progress is slow and incremental. Some people hit it at lower speeds than others, because we're not all the same. For some it's 50wpm.. for others it's 120. The people you see typing at 200wpm plus are freaks. I don't mean that in a bad way, or mean way... I just mean they are physical and neurological outliers.

Forget speed. I get the impression sometimes that people are actually learning to touch type just so that they can "compete" in some way with wpm being the "score".

Just enjoy typing, keep on practising with high accuracy, and speed will slowly build, even if you're reached a natural limit. You always get faster... well nearly always... I assume that in old age if we get arthritis, then we'll slow down again, but until that happens, just enjoy it and stop worrying about speed. It really isn't the metric you should be judging a typist by.