Which PLC are you using. What is the range of fluctuation?
For FMD, Nano-10 and F-series the ADC resolution is 12 bit which means the value fluctuates for voltage variation as low as 5/4096 = 0.00122V.
Electrical noise imposed by the environment via electromagnetic coupling or even component white noise, switching power supply noise etc can cause the fluctuation in readings. A well setup system may still observe some fluctuation around +/- 3LSB which is quite normal. You can turn on the moving average settings in the ADC configuration which will smoothen the readings somewhat. Another way is to take a lot of readings and get the average which will even out the fluctuation and give you the average readings.
E.g.
A = 0
For I = 1 to 100
A = A + ADC(1)
NEXT
A = A/100
A is the average reading of 100 samples.