Missed service because the unit was behind a locked gate. Who pays?

Yes, a missed service visit due to a locked gate is typically billable, and the responsibility for access usually falls on the customer. Your rental contract almost certainly includes a clause requiring you to provide unobstructed, unlocked access to the portable toilets for every scheduled service visit. The driver's time and fuel are incurred whether the service is completed or not, so companies charge a trip fee to cover that cost when access is blocked.

For a commercial build with 40 workers, your setup of four standard units and one ADA-compliant unit meets the OSHA 1926.51 requirement for construction sites. Weekly service is the industry norm, but the placement of the units 75 feet inside a gate makes truck access a critical part of the agreement. The "trip charge, unit inaccessible" line on your invoice is standard practice. To avoid future charges, confirm the gate is unlocked on service days or discuss alternative arrangements with the rental company, like providing a lockbox code.