Everything Printbus and CA said. And:

1) If you were printing in ABS and have now switched to PLA you may have residual ABS left in the nozzle. This will not come out at PLA temperatures. The best way to purge a nozzle is not to remove the previous filament by backing it out of the extruder but rather by snipping it off and letting it run through the extruder. Feed the new filament in as the first disappears. This effectively forces the old filament out.

2) Change/purge filaments at top temp. If filament A runs at 210 and filament B runs at 230 then pick the higher of the two. This should make sense.

3) Poor quality filament will block your nozzles. I had the same problem in my early days, Bought a nozzle, heater block, stripped everything, etc but it kept clogging. Changed filament... no more clogs.

4) Use the correct temps. Again, an obvious one. Too low and you will have feed issues.

5) Over-extruding with too low a temp can also cause clogging. Make sure your extruder is calibrated!

Please give us the following info:
Slicer
Material brands and type
Extruder temp