First of all, the summoning system in Dark Souls does not work both ways. When summoning it is based on the host.
To give an example, a SL 50 can summon as high as a SL 65, but only as low as a SL 35. But if a SL 60 tried to summon, they would only be able to summon as low as a SL 44.
This may be the explanation as to why one person can see the sign but the other can't, you're simply out of the range for one of the hosts to see the sign.
To explain the failed summons, replacing a summon sign is a bad option because of the way the signs work. It's too long to explain, but putting it down too fast will cause problems. That and the fact that other people are trying to summon as well.
Also, your console plays a part in summoning.
Certain Xbox players can not summon each other due to location. The PSN is pretty solid for the most part. GFWL is just hit and miss.