The drought has exacerbated many lawn problems.  Yours may well be a poor soil underlayer that makes it difficult for grass to grow.   You can renovate your lawn this fall, and It's best to first get a good soil test done.  Your soil may be deficient in certain nutrients that help grass grow.  The UMN Soil Test Lab https://soiltest.cfans.umn.edu/  will help you find the soil amendments you need to get grass growing in those bare spots.   Then purchase  a good brand of a turf-type tall fescue and Kentucky Bluegrass mix from a reputable nursery and rake it in.  Keep it watered.  For more information on renovating your lawn, see

https://extension.umn.edu/lawn-care/renovating-lawn-quality-and-sustainability