I've changed few of these. Rohs solder + multilayer ground planes don't make it too easy.
99% of the time it's caused by the iron, lacking power or using a tip head that can't transfer enough heat.
So when you thing the solder is melted, it's only the surface layer.
The copper braid will take heat from the solder joint, so when there're ground planes involved, it's not the best option!
First, use a wide, thick tip, put it on the bottom side, but watching from the top.
Add some solder on the top, keep heating until it melts. Now you know it's fully melted all the way through.
Take a cheap piston pump, and give it a shot! Clean hole with no effort!
Don't be unpatient when heating it up. You want the solder fully melted.
If you only take part of it, like in your picture, it'll be very hard to clean.
Not only that, It'll require a lot of heat, causing a lot of stress on the pcb and risking to damage it.
When that happens, you need to refill it with fresh solder and try again!