I believe Niji explained it quite well how convoy/plunder works...
When you are low level or low power in your server, you tend to be an easy prey.. Noone wants to lose in convoy/plunder.. so people chose others they know they have a high chance of win... Ofc this doesn't work everytime. Since it won't let you close/refresh the plunder results, you are forced to choose whoever showup there, whether weak or strong..

I think plundering is luck based.. if there are too many people stronger than you in your server, have a high chance of losing... and since most of them are done in the first 15 min - you might have a hard time finding a match later for plunder.
But convoy, thats a different story. For this, keep in mind 2 things:
1) Start late.. by then many will be done and your chance to go without being attacked will increase a lot.
2) Try to get someone strong in your group as partner. That way people will be discouraged from attacking you.