You are not providing much details, so I will be generic here too:
- If molecular weight has changed, you will have difference in the flow meters if they are oriffice plates, flow nozzles or annubar. You should be correcting for molecular weight.
- I don't understand the fact that Delta P is "normal" but "head is higher". If the pressure drop is normal, but your calculated head is higher this means that the MW is lower. That is, for the same pressure drop you have more height. In this case, you have an increased pressure drop. Pressure drop chenges with the MW for the same actual rate.
- If the compression ratio is low, what may be the case in recycle compressors, temperature readings can have a big impact on head calculation. Check also the efficiency calculation.
- There may be an error in the calculation. If you are using Excel, it is not difficult at all to introduce an error in a poytropic head formula.