The fluid level does not effect the jack retraction, unless it is being restricted on it's return to the reservior. Typically, hydraulics will initially move slower in cold weather, before the fluid warms up. However, as it is that you are in California, that's likely not a issue.
It could be that your valve is not opening all the way due to a weak solenoid. You may want to try the manual override and see if your jacks retract normally. If so, then the solenoid is most likely your problem. Your owners manual will instruct you on how to do this.
Also, how is the condition of your return springs? Those big springs on each jack are what is "pulling" your jack up to the retracted position.