From your description of the problem there are three likely issues. Either your glassware is not sufficiently clean, your water that you are making the standards or the reagents in is contaminated with aluminum, alkalinity, iron III or some other species that is inhibiting the SPADNS reaction, or your reagent is bad. There are a number of sources available that describe sources of error, diagnostics, and how to prevent them impacting your results.

@John Canham Thank you so much sir

Please look at the following below links which may help you in your analysis:

http://indiawrm.org/Website/Webpages/PDF/-download-manuals-WaterQuality-WQTraining-36MeasurementofFlu.pdf

https://www.epa.gov/sites/production/files/2017-08/documents/method_13a.pdf

Thanks