The fairing itself is large, (a pair can enclose a school bus sized payload) and is very light, and not very aerodynamic when split into two halves.
When joined together it is very aerodynamic, when split apart, dramatically less so.
Thus the parachute has to overcome a very oddly shaped, unoptimized for gliding shape.
It is fairly large, ungainly, and hard to control so that it does not twist nor interfere with the parachute.
In the question about Fairing Separation Altitudes the questioner compiles a table of how high the fairings get released. They all release above 100km altitude and about 7000-9000 km/h. Re-entering from such heights and speeds is very difficult.
That SpaceX has been able to get them down to the surface, to land on the water, visibly intact (Obviously wet with salt water and not reusable, but more intact than they have any right to be), is pretty dang impressive. Now they are trying to hit a net, that is large for humans, but tiny when you compare the height/speed numbers.
They have gotten within 50m on one attempt, and on the Iridium-7 crew reported they could see the fairing, but they failed to catch it. That is pretty darn close.
Expanding the size of the net is one way to make it more likely to succeed. They seem close enough at this point that minor tweaks should make it happen soon enough.