I guess you saw several Roman emperors incapable of producing a natural-born heir and understood it as inability of all the Romans to multiply.Or any other civilization before the advent of modern birth control? How is it possible for several Roman emperors in a row to be incapable of producing a natural-born heir to the Empire?
Well, from all we know the Romans and the population of the Roman Empire in general did not have any problems with reproducing.
Actually dencity of the imperial population reached the peaks which were achieved again only more than a thousand years later.
We are not speaking here about the disastruos effects of plagues, barbarian invasions and Civil wars as having nothing to do with capacity to reproduce.
But (answering your question) - yes, some emperors in a raw had problems with producing a legal heir. And they are surprisingly many.
And if we look attentively some of these emperors did not produce heirs before they became emperors. Which makes us think that it was a bigger problem - some reproduction problems of the Roman elite.
What are the reasons? I do not know.
The Roman marriage in the high society was not necessary based on love, it was more often about choosing a proper girl from a good (Roman) family.
I might guess when a wife was still young and attractive the couple usually had sex and usually the birth(s) took place during first years of marriage. After a few years.... well, the standards of the Roman male behavior were about sex with a lot of partners. And the man was not supposed to have sex only with his wife, it was not obligatory I mean. Actually the husbund did not feel obliged to have sex with his wife at all having a lot of other more young and attractive alternatives.
Bisexuality was a norm and to make things even worse (for reproducing) some of the noble Romans preferred homosexual partners.