What should one do if the slump test fails for a given concrete?

If the slump test fails for concrete then immediately reject that concrete because high amount of water can decrease the durability of the concrete. But dont add water if the slump test fails.

If the slump test fails, then we should reject that concrete.

Because slump test gives an idea about workability and water-cement ratio

If the water-cement ratio is high, then the workability of concrete is increased. Still, the durability of concrete is decreased due to excessive water, which forms voids in the concrete and reduce the strength of the concrete.

Generally slump value is wearing from 25mm to 175 mm it depends on how much workability we required

If slump results from test show shear or collapsible slump, then it should be rejected, and the test should be repeated. Now, if at site RMC is used and slump results are not satisfactory, then we can manage the water-cement ratio by adding more cement content, which will increase strength. Also, we can use admixtures such as plasticizers or superplasticizer to maintain workability of concrete.