I saw in the 1.19 patch notes that were posted recently this little gem.

Players may only pledge their allegiance to a single Grand Company. Once they have done so, players may not do the following:
•Leave their current company or join another.
•Accept another company's quests.
•Use seals earned while a recruit of another company.

So if you've picked one company to be your main, but done the others two's quests and have some seals you should spend them all up before becoming a private. Maybe the crafting books or other consumables?