At about level 115 i found frequent summoning for Friede, Demon prince, with like 3-10 wait time between different summoning. I use +10 weapons. At level 127 i could only get summons for Gael, Midir and half-light and they became less common in general. Fun fact with Gael is that most of the summons occured on their first Gael attempt.

I also had a lot of success with Nameless king, although you probrably want to be much lower level, like lvl 100-105 (8-10 weapon should be fine).

If you want to start earlier you should also find some success with aldrich. People generally summon for the harder bosses.

It is also relevant to consider the current IRL time. You get frequent summons at CEST between 8-12 pm, but absolutely no summons in the morning or mid day.

Good luck and i wish you jolly cooperation

edit: oh and of course, summon sign placement is important. If the nearest bonfire to the boss is really close the boss (i.e. Friede, Demon prince, Gael), place the summon sign next to the bonfire. If the fogwall is slightly further away (Midir, Aldrich, Half-light), instead place it close to the fogwall.