I solder Deans pretty often. If you have the wrong flux it can be a problem. As Don mentioned, you must tin the wires and posts. Sand the posts, flux and then tin{add a little solder. Flux and tin the wire then use something to flattin out the wires. Add heat to the underside of the Deans posts until the tin/solder flows then add the wire and some more solder.
Reason I mention the flux. I was teaching a student how to solder servo wires. Mine were just fine, his looked like hell?? We were using the same solder? I tried to do some wires using his flux and they looked like hell!!! Went back to my flux and they came out nice and smooth.
Don't get the posts on your deans so hot it starts to melt the plastic!!! Just to the point you see the solder/tin melting.
Other Gene