tidyverse: grouped summaries (with summerize)

This code is not correct:
disc_by_month %>%
    group_by(year, month) %>%
    summarize(disc_by_month, vol = mean(cfs, na.rm = TRUE))
It should be:
disc %>% group_by(year,month) %>% summarize(vol=mean(cfs,na.rm=TRUE)

Eric/Avi:

That makes no difference:

disc_by_month

# A tibble: 590,940 ? 6
# Groups:   year, month [66]
     year month   day  hour   min    cfs
    <int> <int> <int> <int> <int>  <dbl>
  1  2016     3     3    12     0 149000
  2  2016     3     3    12    10 150000
  3  2016     3     3    12    20 151000
  4  2016     3     3    12    30 156000
  5  2016     3     3    12    40 154000
  6  2016     3     3    12    50 150000
  7  2016     3     3    13     0 153000
  8  2016     3     3    13    10 156000
  9  2016     3     3    13    20 154000
10  2016     3     3    13    30 155000
# ? with 590,930 more rows

I wondered if I need to group first by hour, then day, then year-month.
This, too, produces the same output:

disc %>%
     group_by(hour) %>%
     group_by(day) %>%
     group_by(year, month) %>%
     summarize(disc_by_month, vol = mean(cfs, na.rm = TRUE))

And disc shows the read dataframe.

I don't understand why the columns are not grouping.

Thanks,

Rich