The small black boost control solenoid (mounted on the intercooler hard intake pipe) is a common failure point, but so are the vacuum hoses. There's a simple test: if the hoses are in good order (you could change yours - 3 metres of 3mm hose from Supercheap will do the job) then connect the vacuum feed across from the top of the motor directly to the actuator and see if boost is produced.
If there's no boost: two possibilities. Either the brake booster has a fault (it is vacuum-driven as well, but not a common fault) - and you'll know it, because the brakes will not be as effective - or the turbocharger has a fault.
If there IS boost - most likely culprit is the BCS. Nissan flog 'em for over $200. You can pick them up on eBay for about $35. I'd normally add a suggestion to look at a part from a wrecking yard but given the price of the eBay ones (and that there's been very few complaints about these above and beyond the ones Nissan supply) there's no point, just go new, or replace with a Tillix or Dawes valve.
The turbocharger fault might be stuck vanes (rebuild/replacement) but it might just be a leak in the actuator's diaphragm too. Test this by getting a new piece of hose, connect one end to the actuator and suck on the other end (yes with your mouth, not with a vacuum cleaner/pool cleaner or a Chery A1 - which sucks more than almost anything else) and watch the arm underneath the actuator, which should move as you create vacuum. If you can continually draw through the hose, it's a leaky diaphragm (has happened, but not often) and replacing the actuator will fix it. If you can't, then the vanes may have siezed (this is quite rare).
Tillix/Dawes valves: personally I have a Tillix valve on mine, it's reaction-driven, no input from the ECU, and you need to adjust it properly to prevent overboosting which the Garrett 2056V can easily do (this WILL engage the check engine light and throw a boost sensor high error - I think it's P0238). If it's WAY too high and the ECU hasn't triggered, it's quite possible (yes I've done it) to pop the high pressure hose off (mine slipped off the intake manifold as we were climbing a hill on the Putty Rd towing the caravan late one night).
Now, with that endorsement and warning out of the way, I have purchased a cheap BCS from eBay (it should be here during the week) because I want to see for myself if they're any good.