Rural Payments Wales says more than one-third of farmers who have so far submitted National Reserve applications have failed to supply supporting evidence.
Of the 363 National Reserve forms received to date, supporting evidence is outstanding on 138.
The National Reserve helps new and young farmers who do not automatically qualify for Basic Payment Scheme entitlements.
CLA Cymru, which has highlighted this issue, said it was one of many mistakes Rural Payments Wales had flagged up.
“Some of the mistakes identified include trying to claim entitlements without declaring the land, and not claiming common land rights when they have in the past,” said Karen Anthony, CLA Cymru policy director.
“They are easily-made mistakes, but it is so important to get it right.
“Farmers must check their acknowledgement receipt to make sure their claim actually covers what they think they claimed. We cannot stress the importance enough – getting this right must be a priority. After all, it is the foundation of payments over the next five years.’’
Mrs Anthony urged farmers to double-check their forms and to make sure the correct supporting evidence is attached. Corrections can be made right up to the 15 May deadline.
“If farmers find they need to change something, they should get in touch with the Welsh Ggvernment immediately,” said Mrs Anthony.