When you flop a set on a monotone board, you must bet aggressively to charge draws and build the pot before the board gets too scary.
While we have bottom set, the monotone texture is highly dynamic. We should lean toward a larger sizing to charge the BB's numerous spade draws and wheel draws.
Note: Sizing is too small; on a monotone board with a set, a larger bet (67% pot) is preferred to maximize value against draws.
Checking back here is a significant mistake. Even with four spades on board, our set is a mandatory bet to get value from any single spade BB might be holding.
Note: Checking back is a major loss of value; we should overbet to polarize and target BB's smaller individual spades.
Once the fifth spade hits the river, the board is a straight flush. Everyone has at least the Ace-high flush. Checking back is standard as we only lose to a straight flush (3s6s).