I have entered event capacity but the figures aren't decreasing when tickets are purchased. Why is this?

This would be because you haven't specified which tickets / products should be included in the event capacity.

Select "Tickets / Products" from your event management menu and then either click the product name or select "Edit product" from the product management to edit the product. Then ensure you have checked "Include in event capacity".

If you only have one ticket / product for sale there is no need to enter event capacity as this will be handled by the stock quantity you have defined for this ticket / product.