Without greater detail, a broad scope of possibilities exist. If you're talking about a situation in which a large change in fuel burn has developed between yesterday and today, it's an entirely different question than a scenario in which over time one engine is simply burning more fuel than the other for the same power output.
As already noted, a dirty compressor reduces efficiency, as does a worn one. Sometimes substantially. Fuel control wear and other inefficiencies can lead to a higher fuel burn. Bleed leaks, stuck bleeds, etc can lead to a higher fuel burn. External fuel leaks can do the same, and aren't as easy to identify as one might suspect; a leak which isn't evident at lower fuel flows may be difficult to identify, but which causes greater fuel losses in flight (that are subsequently evaporated/carried away in the slipstream) are tough to find (as opposed to oil leaks or hydraulic leaks, which leave more evidence).
Exhuast inefficiencies are also possible, as any obstruction, no matter how small, in the exhaust stream affects airflow through the engine, and consequently affects efficiency and fuel burn.
Leakage internally or externally in the fuel system may also be a culprit, with differences between the engine fuel system and aircraft fuel system as to how the leak is identified, etc.
Engine-side fuel leaks or inefficiences can range from internal bypassing to leaks at fuel lines or fittings, to failures of flow dividers, pressurization and dump, or nozzle errors (though typically nozzle errors are more of a spray pattern or blockage problem, rather than ones that increase fuel flow to excess).