Your options order is most likely receiving a partial fill because it has low liquidity in the market. Low liquidity means that not many people are trading the contract when you place the order. Partial executions occur when there are not enough matching orders to fill an entire order at the specified price or better.
Keep In Mind
There must be a buyer and seller on both sides of the trade for an order to execute, so your one order may be filled in multiple, partial, executions. Moreover, your order may simply go unfilled if both a buyer and a seller cannot be matched even for partial executions.