If there is no spark or the spark is very weak you may need to adjust the piezo device.
A piezo ignition has a small crystal, when it is squeezed the crystal creates a high voltage current which causes the spark. Over time the crystal will need slight adjustment to maintain the spark.
It is important to remember not to crush the crystal. So only make small light adjustments.
On the engine there is a cam driven from the crank of the engine. This pushes a metal lever connected to the piezo. When this lever is pushed it puts pressure on the crystal and a spark is created. The spark will only be created if there is an electrical connection. If the spark plug cable is removed or not connected properly it may not spark.
There are 2 adjustments. On top of the piezo is a nut and grub screw. The grub screw has an allen key socket. Undo the nut about a turn or two. Now with the allen key in place turn slightly. Maybe about ΒΌ of turn (tightening). I suggest leaving the allen key in place while doing this.
Rotate the flywheel CAREFULLY. Keep your hands aware from other parts so not to get a shock.
You should get one or two sparks each rotation (varies on the model).
Use trial and error to tighten slightly and check by rotating the flywheel. DO NOT OVER TIGHTEN.
It is better to get it working and undo slightly to find the minimum amount of pressure.
Once you have it working HOLD THE ALLEN KEY IN PLACE AND TIGHTEN the nut. Then rotate the flywheel to check there is still a spark.
There is also a second adjustment. This adds pressure to the crystal but also slight adjusts the timing. There is a second grub screw and nut on the side of the piezo through a metal plate. By adjusting the grub screw here you change the angle of the piezo very slightly and also the pressure slightly.
This is NOT the primary adjustment. I have rarely had the need to adjust this. I suggest avoiding.