Amplitude drop as frequency increases on function generator
If it is an older analog function generator without a calibrated amplitude this may be normal.  More modern wave generators should have flat amplitude output. The output impedance is usually 50 ohm. If you have a frequency dependent load like a short cable and an oscilloscope with a high impedance input then this is also normal.  The function generator is doing it's thing properly but your load is a capacitor.  Try setting your scope to 50 ohm mode or using an external terminator.