May be the DNA destroyed in this period or your primers contam you can check the DNA by use another primer

Dear Ahmed The Sample in the same run had also , other 4 genes screening

They all give Amplfication plot 

so you can screen about mutation in the primer annealing region

Can it be  two samples ?

Does this finding go with Sample evaopration?

As Ahmed says a polymorphism under either primer will result in no amplification or less likely a deletion of the amplimer or primer binding site  ( homozygous if on an autosome) will also result in no amplification