If your vehicle is showing factory or aftermarket boost gauge shows lower pressure than normal, here are the most likely causes and the parts you'll need to fix each one.
Pressurized air is escaping through a loose clamp, torn silicone coupler, or cracked intercooler pipe before reaching the engine. This is extremely common, especially after recent maintenance or modifications.
The vacuum line feeding the mechanical boost gauge is kinked, pinched, or leaking. If the gauge is electronic, the boost pressure sensor may be failing, unplugged, or reading incorrectly.
The blow-off valve is opening prematurely or failing to seal under pressure due to a weak spring, torn internal diaphragm, or improper adjustment.
The wastegate actuator is opening too early because of a weak internal spring, a torn diaphragm, or incorrect preload adjustment, allowing exhaust gas to bypass the turbo.
The electronic or manual boost controller is failing to bleed off the correct amount of pressure to the wastegate, or the vacuum lines to it are routed incorrectly.
Exhaust gases are escaping from a cracked manifold or blown gasket before they can spin the turbocharger turbine. You will usually hear a ticking noise under load.
The turbocharger itself has worn bearings, damaged compressor/turbine wheels, or excessive shaft play, preventing it from generating adequate boost.
A severe restriction in the exhaust system prevents the engine from breathing properly, which in turn prevents the turbocharger from spooling up and creating boost.