What could cause the trailer to not raise?

The reason the trailer might not raise is due to the hydraulic power unit (HPU) not being connected. The HPU is critical in supplying the necessary hydraulic pressure to lift the trailer. Without a proper connection to the HPU, the hydraulic fluid cannot be pressurized effectively, preventing the trailer from being lifted.

In contrast, having a proper hydraulic fluid level, a fully charged battery, and good tire pressure are all supportive factors that contribute to the overall functionality of the lifting mechanism, but they do not replace the need for a connected and functional HPU. If the HPU is disconnected or malfunctioning, the hydraulic system will not operationalize, regardless of those other elements being in good condition.