An empty-looking cell is not always a legal move. In Ringzzle, space belongs to a ring size: each of the nine cells has one small, one medium, and one large slot.
Look at size before color
A large ring can surround a small ring already on the board. A second small ring cannot go into that same small slot, even if its color matches. When a drop returns to the tray, check the size slot before trying again.
The three tray positions do not mean small, medium, and large. Pieces are generated individually, so you may receive two or three of the same size.
Plan the whole tray
Look at all three pieces before placing the first. You can use them in any order, and the tray refills only after all three are used.
For example, if two tray pieces are large and only two large slots are open, placing a small piece in an unrelated cell will not create extra room for those large rings. Look for a color clear that can reopen the size you need.
Prefer moves with a second option
As a strategy, try to place rings where they can contribute to more than one possible line. The center participates in a row, column, and both diagonals, but it still has only three size slots. Filling it without a clear in mind can make later moves harder.
Keeping a slot open is an option, not a guarantee: future pieces are random. Choose the move that leaves you useful choices with the tray you can already see.
When does a run end?
Game over occurs when none of the remaining tray rings fits anywhere. One blocked tray piece does not end the run if another piece can still be placed. That playable piece might complete a clear and make room again.
Next, learn why different-sized rings can clear together, or return to Ringzzle and try planning one full tray.