Sorry about that. We do not (intentionally) show ads before every trailer. Video ads should be frequency capped such that you only see them occasionally.
Ironically, ad / cookie blocking plug-ins can interfere with the frequency capping itself so if you try disabling any ad blockers, the trailers should work as expected.
If this does not fix it, have you selected the option "Do Not Personalize Ads from Amazon for this Internet Browser" on https://www.amazon.com/adprefs ? By selecting this option, Amazon would not apply frequency capping on your requests, and this would explain the behaviour since you are essentially opting out of us storing any information, including whether or not you have already seen the ad.
If none of the above works, please let us know the system and browser which you are using plus also the country from which you are accessing IMDb.