When using Party Finder, try shout in the popular areas. If it's not a very off peak time, this normally speeds up people joining.

I normally wait 15-20 mins, then try shouting. All Heavensward areas + 2.0 main cities + Mor Dhona + Popular FATE locations like Northern Thanalan and Costa Del Sol.

Repeat shouting every 5 mins or so and it normally speeds up getting a full party. And don't give up after 25 mins, just let the full party finder queue time of 1 hour expire.