it could be a couple of different things. you should have 2 vacuum relays under the nose, one for each side. could be the one that controls the driver's side (personally, not what i think, but it is possible).
check the vacuum hose ends where they join the nipples on the relay and on the actuator. they have a tendency to dry out and crack on the ends. that would obviously cause a leak (but might not be enough to "hiss". since they are just slow, i would doubt you would be able to hear anything over the engine running as it is a slight loss of vacuum, not a complete failure).
if your vacuum hoses are a bit older, they may have dried out a bit from the heat and have micro cracks or are porous enough to be leaching a bit of vacuum and it is preventing the headlamp actuator to operate at full capacity.
it could be the actuator seal as previously mentioned, but the actuator itself could be going bad and the diaphram inside is starting to fail.
hope some of that points you in the right direction to solve your problem
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