Playing in Time - Listening
Playing in time is challenging, and when you think your timing is good, then try playing with a metronome. The skill you're developing is a listening skill. You listen to the metronome (which is a stand-in for another musician) and adjust your playing to stay in time with it. Just like when riding a bike with a friend and trying to hold a conversation, you're consistently adjusting your speed so that you're within comfortable speaking/hearing range. As soon as you lose concentration, you're soon too far behind/ahead to continue the conversation.
Your tip today is that playing in time is about listening... more to the other musicians (or metronome) than yourself. If you're too wrapped up in what you're playing, you can't play in time.
For students learning a new piece of music (such as the Solo Harmonica Study pieces you submit to me playing to a metronome)... if you find that you have to really concentrate on playing the correct hole numbers and techniques, then you need more reps, you're not ready to perform the piece. When you can stop focusing on the mechanics of performing the piece, then you can really dial in your rhythm and dynamics. This is not to say to ignore the rhythm of the licks you're learning in study, just don't expect to be able to play them well in time until you've mastered the mechanics of their performance.