Edit: It looks like the argument here is that the US is not calling for an instant ceasefire, but instead saying that one is very important to have. China and Russia say it should be immediate. The US also tied it to hostage talks.
Another resolution is in the works to call for an immediate ceasefire, but the US is expected to veto it because they believe it could endanger hostage talks.
Historically ceasfires have been used by Hamas to resupply rockets for the next rocket barrage on Israeli civilians. A ceasfire without hostages being released would be nothing more than a failure on the Israeli side, so would not be accepted.