I bought a slot and it says I am not using it

A slot is bought for a named hour and is spent by a rotation that happens to touch that hour. Those are two different things, and nothing joins them automatically: you choose a frequency, every departure after the first follows from block time, and the only lever you hold is when the aeroplane's day starts.

So it is entirely possible to hold 06:00 at a field and never fly in it — and after thirty days under 80% use, the slot goes back to the pool.

Use the aiming tool on the slot desk. Give it the hour you hold and it tells you which day starts put a movement in that band, or says plainly that none do. Worth knowing before buying rather than after.

Two things catch people out. A rotation touches an hour at both ends — a departure and an arrival each need a slot, at their own airports. And on a route whose round trip runs past midnight, the day's start can be pushed later by last night's arrival, which moves the hours it lands in; if that happens, aim it again.