Lawn Grass Question number 1001

Ok there’s probably now umpteenth grass questions and queries. But here’s mine no apologies. New lawn down 3 years this summer about 4000 sqmtr robot polices the lawn daily between February and November. I have yellowish patches on grass the tips and undergrowth. Some of the lower grass dies but the top section continues to grow. Drainage is good I know fertiliser maybe be required how much how often?… any help advice appreciated