Four shops sold knives to children during a police test purchase operation.
A total of 26 shops across Nottinghamshire were visited on Wednesday by young people posing as customers trying to buy a knife.
Most shops stuck to the law and refused to sell the weapons when they could not prove they were aged 18 or over. These shops will each be sent a letter updating them on the operation and thanking them for supporting Nottinghamshire Police's efforts to tackle knife crime.
However, four shops did not ask for ID and sold knives to the young person and have been informed follow-up action will be taken.
Chief Inspector Donna Lawton, knife crime lead for Nottinghamshire Police, said: "In total there were four failures, which were all smaller private hardware shops. This is different to previous years which has seen a mix of larger national chain shops and privately run shops.
"All were re-visited by officers immediately after the failure and the illegal sale highlighted to staff. Staff members making the sale were identified and informed that follow-up action would be undertaken.
"During the last Operation Sceptre in September 2018 we carried out 22 test purchases and there were five failures, so this is definitely an improvement but it is still disappointing that we have had some failures again.
"We rely on shops and supermarkets recognising the seriousness of selling weapons to young people and understanding that they have a big responsibility in helping to prevent knife crime.
"Knife crime is not something the police can stop on our own. We all need to work together to protect each other."