Timing of text, etc

Okay, have been teaching myself Motion and am wondering why I'm regularly running into timing issues.

Example 1: Text

I have created a number of text layers in my project and set the timing of some bullet points separately as I used separate layers for them. I had them set to appear 2 seconds apart and to disappear at the same time. This all worked fine until I decided to change the time later on. I did this, but now for some reason Bullet Point 2 appears first, followed by bullet points 1 & 2 at the same time and bullet point 4 last. Why is this happening? I have checked the timing properties of all of them, as well as of the groups they're in. I've even changed the order of the layers, but this made no difference. Everything looks okay...but something's amiss?

Example 2: Timing in general

Sometimes when I try to change the timing of a layer in the Properties it won't allow me to change it (but why?). I can then change it manually by dragging the slider in the Mini Timeline, but I can't get the slider to go exactly to the individual frame I want. (Is there way to do this?) But usually once I've manually changed it in the slider, I can then go back to the timing properties to change it exactly. Why is this happening? I can't seem to work out the pattern here?

Thanks for any help!

Kindest regards,

Joe.