The manual piezo is the battery part of an electrical circuit, the ignition wire down to the burner is the positive lead and the fire and fire front the earth return back to the piezo/battery.
Like any other circuit, if this circuit is not complete then you will get a poor or intermittent spark or no spark at all.
The piezo has a direct connection at the bottom for the ignition wire, he earth is a brass tab sticking out to connect it to the metal of the fire front. I'm not sure but this tab might simply need to be touching one of the grill louvres, you should be able to see this with a torch looking through from the front.
Theres then a spring wire, by the side of the gas valve, to connect the fire front to the base plate of the fire itself, you just might be able to feel this under the bottom edge without removing the front.
To remove the front, the top pulls out off two clips, one clip either side close to the top, again the clips are visible with a torch through the grill
Other than that a faulty piezo can give the same symptoms and if it is at fault, it's a simple job to replace.