Not all
If DA revolver cannot act SA, it is not DA but DAO.
DAO means that the weapon can only be fired in double action mode. Examples include the DAO variants of the Beretta 92 and SIG P226 DAK, or it can also sometimes refer to striker fired pistols such as Glock or M&P that are designed with a much heavier trigger pull (such as some police departments like the New York Police Department require). It can also apply to revolvers that have a covered up hammer so they can only be fired in the DA mode only, such as the Ruger LCR or Smith & Wesson 442/642.
DA/SA refers to weapons, and usually semi-automatic pistols, that can be fired in DA or SA mode. The first trigger pull is long and in the DA mode, while all subsequent trigger pulls are in the shorter and crisper SA trigger mode, until the weapon is either empty or safely decocked. Examples include the Beretta 92, SIG P226, Walther P99, and the HK USP. Most DA/SA are hammer fired, but exceptions apply, as a few striker fired pistols like the P99 are DA/SA.
DA is typically used to refer to double action revolvers that can be fired in double action or have the hammer pulled back for single action, and in modern cases, with a swing out cylinder. This includes guns like the Ruger GP100 or the Smith & Wesson 686.
SA is typically used to refer to single action revolvers that can only be fired in single action mode after the hammer is pulled. This includes iconic revolvers like the Colt Peacemaker (AKA the Colt Single Action Army) or the Ruger Vaquero.
If you don't believe me, hope you believe him:
https://www.youtube.com/watch?v=V2be4eS_tMI