Why was only part of my order executed?

This is almost always a liquidity issue: the market simply didn’t have enough sellers (if you were buying) or buyers (if you were selling) at your specified price to match your full order size at that moment. It’s more common in mid-cap and small-cap stocks with lower trading volume, and in options contracts far from the current market price, where fewer participants are actively quoting. It can also happen with a large order in an otherwise liquid stock if you’ve set a tight limit price that only a portion of the available quantity matches. You check the order book to see the filled quantity versus the total quantity you placed, and you decide whether to let the remaining quantity stay pending, cancel it, or place a fresh order at a slightly adjusted price if you want the rest filled sooner.