Make sure the temperature is high enough, but not too high. A very high temperature will encourage quick formation of oxides which can be much tougher than just metal.
Shock said, "Once you remove the solder remove the tip from the pcb then let go of the vacuum button.". Absolutely correct, but I would state it a little differently: Let go of the vacuum button ONLY after you remove the solder remove the tip from the pcb. This allows some air to move through the tube and remove any molten solder trapped inside. If that solder sits there, it will oxidize and become hard and nearly impossible to melt.
In extreme cases of clogging, you might need to drill the blockage out, followed by cleaning with an appropriate pin. Have a look at airbrush tip cleaning tools, welding torch tip cleaning tools, or carburetor cleaning tools. Any of those can be had for just a few dollars on ebay or wherever you like, and will have multiple sizes of pins that work well for this.
FWIW, I have desoldered many joints with my Metcal MX-DS1 and have only clogged the tip once or twice - ever. I routinely do dozens to hundreds of joints successively without any problem. This had never been the case with my previous electric pump driven tool. The Metcal uses a venturi powered by compressed air to generate vacuum. The heating power of the tips is amazing too. I have desoldered through-hole electrolytic capacitors from computer motherboards (probably well over 10 layers) without much grief. Not many tools could do that in the hands of an amateur like me.
edit:
I'll add that you should make sure that the filter(s) are not clogged up which will greatly reduce air flow. These clog up with fumes from flux, etc., and you can't necessarily see the problem. Replace or try cleaning with IPA (soak/rinse/dry).