relative v absolute
One problem you might be having is with the small valve openings. This goes back to a question I asked earlier about whether your +/- 5% was relative or absolute.
Your code shows that you are using a relative 5% error. That is, if you are opening the valve to 5% of it's full 0-16383, you are expecting the feedback to be within .95*5 to 1.05*5 = 4.75 - 5.25% of it's full range 0-16383. If you were using an absolute error, then you'd accept a feedback of 0 to 10% for an output of 5%. If your feedback sensor is no more than +/- 1%, and you are looking for a range of +/- 0.25%, then you'll have trouble.
At the full open (100%) position, relative and absolute error checking are the same. But for anything less, relative error is a much tighter spec, possibly impossible to achieve (at output = 0%, relative error says that the feedback must be 0.0000%, while absolute allows -5 to +5% open.
The other thing that seems to be biting you is the scan time and the signal time. Just as the valve has delay characteristics, so does the PLC. Even sending the output direct to the input, the module still has to measure the new voltage, and the PLC will probably have to do at least one scan of the code before the new feedback signal is read. What's your scan time. How does it compare with 25 msec?
And why 25 msec? Do you have that critical of response rate for your valve? Or are you just trying to confirm that the valve has, in fact done SOMETHING in response to the output?