sen0605 soil sensor NPK

I have used the Soil sensor Sen0601 Temperature Humidity and EC with good results. 

But the soil Sensor sen0605 N&P&K  gives all zero. I have placed it in wet sand with fertilizer 20-20-20 starting form one spoon going to 5 spoons and still I get zero results.

Below is a sample printout from your example program with some added print statements for troubleshooting.

10:16:09.200 -> 1

10:16:09.200 -> 3

10:16:09.200 -> 2

10:16:09.232 -> 0

10:16:09.232 -> 0

10:16:09.232 -> 184

10:16:09.232 -> 68

10:16:09.232 -> 0

10:16:09.232 -> 0

10:16:09.232 -> 0

10:16:09.232 -> P = 0 mg/kg  

10:16:10.329 -> 1

10:16:10.329 -> 3

10:16:10.329 -> 2

10:16:10.329 -> 0

10:16:10.329 -> 0

10:16:10.329 -> 184

10:16:10.329 -> 68

10:16:10.329 -> 0

10:16:10.329 -> 0

10:16:10.362 -> 0

10:16:10.362 -> K = 0 mg/kg  

10:16:12.422 -> 1

10:16:12.422 -> 3

10:16:12.422 -> 2

10:16:12.422 -> 0

10:16:12.453 -> 0

10:16:12.453 -> 184

10:16:12.453 -> 68

10:16:12.453 -> 0

10:16:12.453 -> 0

10:16:12.453 -> 0

10:16:12.453 -> N = 0 mg/kg  

Can you help?

Thank you

Nicos M Ioannou