What exactly have you tried? I suspect you haven't read all the topics that I advised.
You should use FDM menu Options | Settings | Downloads | Network and choose your own manual settings for connection speed. Only then you can change settings for traffic usage modes and section number in FDM menu Options | Settings | Downloads | New download after clicking "Advanced" button.
At first, I upgraded FDM ver.3.5 to 3.8 (without uninstalling). FDM could not download 2 or more items at the time.
1. Then I changed options in "Settings->Network" dialog.
Connection speed: 10MB/s.
Settings for traffic usage modes: Heavy mode.
Maximum number of connections: Set to 2
Maximum number of connections per one server: Set to 2.
Maximum number of running downloads: Set to 2
2. I checked that traffic usage mode is heavy mode in the menu "Options->Traffic usage mode".
However, FDM downloads only 1 item at the time as usual.
3. I checked "Downloads->New Download->Advanced" but I cannot find any cause about the number of downloads and I didn't change any options.
(Before, I have tried to check "Connection->Ignore all restrictions" and "Proxy->Get proxy settigs->Don't use any proxy", but the situation does not changed)
Don't forget that many sites use their own connection limits and you cannot change it even if your FDM settings allow to use many connections and sections per server.
It also might be one of the causes.
Network management in FDM has been changed in FDM 3.8 build 1142. With automatic networking management FDM needs now about minute to determine connection speed and open more connections (create more download sections) if it's possible.
What that new version would not work unless determining connection speed automatically?
If so I try to continue to use new FDM for a while.
Thank you Usher.