have you entered the Company of Champions, which one of the perks of this covenant is restricting summoning to increase difficulty. You can leave the covenant at any time by talking to the Cat, Sweet Shalquoir in Majula .
Are you human? You must be 100% human to see signs.
Did you burn a Human Effigy in a bonfire? This puts you into "offline mode" for an hour so you you can be human and not invaded.
Do you have Name-Engraved Ring equipped? This will prohibit you from seeing any signs but those with that ring equipped (even NPC phantoms)
After you place your summon sign and the summon fails (usually bad connection or the host already has enough players), the White Sign Soapstone becomes unusable. To fix this glitch, quit the game and continue.
Is the area boss alive? If the area boss is dead you will only be able to see Small Soap Stone signs.
¤There are certain areas that you cannot place a summon sign or summon other players. One example is in Hedie's Tower of Flame, after the second bonfire, you go into an area inside the tower, and can not place your sign nor summon other players.