I had some solder joints on the fans that went dry. I pulled them out and reflowed the solder (with an 80W soldering iron mind).
Check your getting 12V to them when they should be in cycle to eliminate the fans themselves, then follow continuity back to the fuses etc.
try bridging the 2 terminals on the fan cut-off switch (very top left of the radiator)..... this should switch the fan on - if it comes on, then it may be that the switch is knackered and only coming on at a higher engine temp, or if it doesnt come at all, then replace it as a matter of course
dont forget that if you have 2 fans (then you have air conditioning), only the fan on the turbo side is for normal cooling, the other one (left) is only for the air con