[Justin Levine]

I had this same problem recently. I confirmed that Overlays and Dynamic Guides were turned on. I also had the snapping button on – but still, the dynamic guides would not appear.

I eventually tried something random that ended up working and fixing the problem: Delete your Preferences for Motion 5.

For Motion 5.1.2 (or later versions):

Quit Motion.
Hold the Command and Option keys while opening Motion.
A window appears asking you to Continue opening Motion or Delete Preferences.
Click Delete Preferences. This command removes the preference files and resets Motion to default settings.

After I did that, the Dynamic Guides returned and everything worked fine.

I hope this helps.