This could happen for various reasons; - the first thing to check is that you haven't disabled autoplay in
Dashboard > Music Stores > HTML5 > (select your configuration). This can be set for each store, so make sure you have it enabled in the correct configuration
Assuming that is ok, it will be browser restrictions that are blocking autoplay. (At the time of writing - Jan 2019) browsers are beginning to block autoplay completely, so you will see this working less and less for all stores.
Google Chrome for example, have set parameters that must be met before autoplay works - e.g. how many times you've visited the page, how you've interacted with it, etc., so if these are not met by the visitor, autoplay will be blocked.
It's likely that in future autoplay will be blocked completely.
Please note that autoplay doesn't work on Mobile or Soundclick, and never has.