Governor time lag; incorrect or overly sensitive speed droop or compensation settings; sticky steam/fuel valves; incompatible or uncompensated response characteristics between generators driven by different types of prime movers; incorrect set-up of companion governors on the same system [ such as mistakenly selecting more than one as isochronous, aka "the swing set" ]; trapped air in the hydraulic systems that drive a given unit's fuel valves / steam valves / wicket gates; and the list goes on . . .
what type of prime mover?
(turbine - steam, combustion, hydro, wind. piston engine)
what type of load?
( massive interconnection grid or small island)
what type of governor?
(mechanical, anolog, or digital)
OEM?