The primer bulb on your Ryobi trimmer attaches to the return fuel line, which runs from the carburetor back to the fuel tank. When pressed, this bulb uses suction to draw fuel into the carburetor of a cold engine. Since suction needs to remain constant, any air leaks in the fuel line due to faulty connections or degradation of the plastic lines will cause your primer bulb to stop inflating.
If the primer bulb is not inflating, you may notice several other symptoms that tell you air is entering the fuel lines from somewhere. You will first notice that the trimmer doesn't start as readily and may require more priming on a cold engine. Once extra air is present in the line, the carburetor will no longer deliver the precise air-fuel mixture needed for the engine to run smoothly. You may notice a drop in power under full load. That is, when you accelerate the trimmer fully, the engine doesn't reach maximum power and the rpm drops off rapidly. If this continues, the engine will just shut off to prevent overheating and damage to the piston and cylinders. If you notice these symptoms, shut the Ryobi off and check the entire fuel system, beginning at the tank, moving to the carburetor, back down through the primer bulb and back to the tank. Look for holes in the fuel lines and loose connections between the different sections of plastic lines.
Since some fuel remains in the bulb, it will degrade the plastic of the bulb over time. This can cause the bulb to lose its ability to inflate. After one or two seasons -- sooner, if fuel was left in the tank more than 30 days -- the fuel lines and primer bulb will need replacing. Movement of the engine during operation also can bump a line loose, causing loss of suction.
Replacing the primer should be done concurrently with replacement of fuel filter and fuel lines. These parts all work in a closed system; when one starts to wear out, the others will have problems too. When disassembling the primer bulb and fuel lines, you should carefully note where each line connects to the carburetor and through which hole it enters the tank. This will help you to install the new parts smoothly and ensure they are in the correct position.
Continue monitoring the primer bulb and fuel lines during normal operation. If you still have suction problems after replacing these parts, you likely will need to remove and clean the carburetor and check the function of the fuel pump.