Depends on how often you do the tests. There are always upticks and downticks in averages when you play a lot of games. Or, if you don't play a lot of games, it could be lack of practice.

I find that my speeds improve if I have a clear mind. Sometimes I can get 20% faster if I just get up and walk around a bit, to clear my mind.

Try going for accuracy rather than speed.... don't know if this helps, but it helped me.

It can depend on how tired, or how mentally tired you become. More tests obviously mean getting more tired, but the next time you start you should be a little better. I think it's called a learning curve. You have ups and downs, but your WPM is steadily increasing if you do enough work. I used to type around 30 when I started, then 40, now I never drop below 50 WPM, no matter how many typos I make. And yes, it can be frustrating mastering the 40 at last and then dropping to 37, 36 on some quotes, but a month later you might never even get below 40 no matter how badly you think you're doing. My advice: Don't worry about small failures and keep practicing. Being 'bad often' is a part of the path towards becoming good.

Should have kept my mouth shut. I looked at your profile 'enyaloth' and wow! Great speed. Don't take any advice from the likes of me!

It's just fatigue really. Take a break, and see your speed go up again.
Shortish breaks of ~20-30 mins, and longish breaks of ~1-2 weeks from focused typing both help.

Naw, I appreciate the reply regardless! Still good insight, thank you :)

I find it helpful to listen to relaxing music while I key. (ocean, birds, even something classical.) Also I have found that if my skin is dry it effects my speed and accuracy. I keep hand lotion close by at all times :)