Steam flow control valve

HI  

I have a flow control valve for steam, where the trend across 4 months period shows that the valve opening is different, for the same PV. Meaning the valve opening is going from 64% up to 90 something % for a steam flow of almost constant around 70t/hrs. Any reason why such variation in the control valve ? If there is a problem , any way to troubleshoot this and what parameters to look at ? 

Thank you 

Your thoughts please 

Thank you