Can't Summon Sixth Horrid Tentacle

[SignedName]

I tried summoning a Horrid Tentacle, and there was a message saying I summoned it and 15 MP was deducted, but no tentacle was actually summoned. If it's relevant, I added names in the character settings page for tentacles 6 times (all the same name, but it didn't seem to actually summon past the first one with a name).