[exim] manual delivery vs. normal queue runs

Hello,

I find that quite often some emails that are not send during normal queue
runs get sent when I run exim -d -M <messageID>. Why would that be? I mean
I understand that sometimes it may happen because a server was slow to
respond, etc.

The regular queue run reports: retry time not reached for any host

and manual deliver is often successful. The same applies to yahoo emails.
Runing exim -Rff yahoo.com helps me deal with in-queue messages which
cannot be delivered during normal queue runs.

I do not think this is exim's fault but me knowing too little about this
great MTA internal workings. Any suggestion, explanation & advice greatly
appreciated.

Thank you!

--
Zbigniew Szalbot