Hi Lianne,
Its difficult to know without seeing your actual project file - as it could be so many things causing this to happen. Maybe you can share your Story file here?
If not, there are some things that you could check. This behaviour you are describing is not "normal" Storyline default, so it must have something somewhere changing the Next button behaviour.
I would first check that there is no trigger on a slide(s) that says something like "Change state of Next button to disabled when the Timeline Starts on this slide".
(dont forget to check the Master Slide - CTRL-F4 jumps to the Master slide - because triggers like this can also appear there)
If you have a trigger like that, then disabling or deleting it should return your Storyline to normal Next button behaviour.
