It's not easy to confirm the problem, we have some solution suggestion for your try.
1.Firstly, let's figure out whether your testing equipment has problem, pls take a photo of the testing equipment,
make sure the testing equipment is for 1-phase, power factor 1.0 not 0.8, some equipment is 1-phase but power factor is 0.8, so the genset output cannot be 100%.
2.If the testing equipment is no problem, we should adjust the engine, make the frequency between 53~54Hz when 0% loading,
so the frequency will be around 50Hz when 100% loading(15kw), see attached photo. Take photo of the 3-meters after engine start but with 0% loading.
3.If the adjust doesn't work, the problem should be the engine, there're many possibilities, this genset was delivered to you about 2-years ago,
so there might be too many incrustation inside the cooling tube of engine, which affect the engine power.