The more complex a system, the more likely it is to break. The trim and tilt system of a modern outboard motor has its own set of problems, most of which can be solved by adding a set of O-rings or swapping out parts that can be unbolted from the system and replaced with parts known to be good, which are then bolted into the system.
Check the manual release valve, located on the manifold, usually in the centre of the manifold. The manual release valve is the joker, the trickster of the tilt and trim system. All potential problems with the system may result from the manual release valve not being closed, or being partially closed, so make checking this valve the first step you take when going after a problem with the tilt and trim system. The valve is also the easiest of the system's parts to swap out. Before swapping any other part, swap the manual relief valve for one that's known to be good.
When the unit won't move up or down and the manual release valve is in order, check the electrical power supply, starting at the breaker box and working your way to the butt connectors at the motor and from there to the ground. If the circuitry is in order, become a parts-changer: swap out the hydraulic motor for a hydraulic motor you know works. If the problem persists, swap out the hydraulic manifold assembly for one that's good.
Speed variations and leaks
If the tilt trim unit moves slowly in one direction or if the unit's hydraulic cylinder -- the piston that moves the motor -- leaks while the system is moving up, swap out the hydraulic manifold assembly for one that you know is working order. Only do this after checking the manual release valve and, if necessary, replacing it.
Won't hold trim
If you find that your motor won't hold trim in either direction, or if you see a small leak around the seal of the hydraulic cylinder, install a new hydraulic manifold assembly. If the problem goes away, great. If the problem persists, however, install an O-ring set for your particular model. If the symptoms still persist, swap out the hydraulic motor for one known to be good. Here too, remember to check the manual release valve first.