Hi Pat
If he was born in March 1938 then, allowing 6 weeks to register the birth, he is probably the one registered in the Apr May Jun quarter of 1938. Did you supply the correct district and reference number?
If you did supply all the relevant info exactly as it appears in their index then email them and I'm sure they will try to sort it out for you.
You wouldn't have got a certificate for one registered in the March quarter if he was registered in the June quarter because that wouldn't tally with the information in their indexes.
Best wishes
Casalguidi