[freeflight]

Here's the actual take by the OPCW on what happened in Douma to prevent their FFM because your WaPo article doesn't have anything official by OPCW: https://www.opcw.org/fileadmin/OPCW/EC/M-59/en/ecm59dg02_e_....

"The United Nations Department of Safety and Security (UNDSS) has made the necessary arrangements with the Syrian authorities to escort the team to a certain point and then for the escort to be taken over by the Russian Military Police. However, the UNDSS preferred to first conduct a reconnaissance visit to the sites, which took place yesterday. FFM team members did not participate in this visit.

On arrival at Site 1, a large crowd gathered and the advice provided by the UNDSS was that the reconnaissance team should withdraw.

At Site 2, the team came under small arms fire and an explosive was detonated. The reconnaissance team returned to Damascus."

The decision to delay the FFM team's visit wasn't made by Syrian or Russian authorities, but by the UN personnel tasked with protecting said FFM team. On Saturday they did finally conduct their FFM and took samples, so there's that.