As long as it is prime eligible (not all items are, but a massive portion of the site is), then yes - it is to be 2 day delivery.
If you have had more than 1 or 2 issues, if you open a ticket to complain - often they will issue you free months for prime. Not a lot of value if you only use prime for shipping and this is your primary concern, but if you use other services (Prime Video for example), then there is at least some value there.
Either way, I'd suggest complaining if you are referring to Amazon prime eligible items. Also I don't know if it's possible, but if there is a way to set your default shipping to something other than Prime, then that may be the culprit.