Simonides claimed he ran out of room on Hermas, but as Snapp drilled Avery with his "you're no Jack Kennedy" in their debate, Simonides only said that because he thought there was no more of it [Hermas] .......
The Shepherd of Hermas is the single longest document of the Apostolic Fathers. Nobody, ancient or modern, would suppose it could completely fit the relatively few surviving pages of Hermas. If Simonides (or anybody) was working with a limited number of blank pages there are quite a few documents of the Apostolic Fathers any one of which would have fitted completely in the surviving pages of Hermas. For all I know, other Apostolic Fathers documents were also in the Sinaiticus but now are lost with the missing pages of Hermas.