Antibody levels following vaccination and infection are thought to decline over time. This is natural and occurs with responses to all pathogens. They don't just fall off a cliff so that one day a person has antibodies and the next not. It also depends on a person's own immune system, how it reacted to the first and second exposure, how its antibody levels hold up in general, how much an individual uses current antibodies to fight infection and how much T and B cell responses, etc.
A decline also occurs in antibody levels of people who received the mRNA vaccines. Although it is generally thought that on average they originally made higher levels of antibodies on receiving the vaccine. I'm unaware of any studies where the participants were carefully controlled for other factors such as age, general health, strength of immune response in general and so on.
Immunity also depends on other factors such as T and B cell responses, which may be greater or lesser after mRNA or AZ vaccines. It's quite possible that the difference in immunity over time between different individuals is greater than the difference between vaccine received.
A sample of one is not much use, but I paid for antibody tests shortly after my second vaccine (AZ) and after 6 months, just before my booster. I can't find information about what are 'good' levels, but, although they had reduced, I still had enough antibodies after 6 months to reassure me.