From the heroku doc:
The logs command retrieves 100 log lines by default. You can specify
the number of log lines to retrieve (up to a maximum of 1,500 lines)
by using the –num (or -n) option.
$ heroku logs -n 200
So probably you need to request more lines with -noption.
As per comment received, you can also stream the current log with:
$ heroku logs --tail
Please look at the doc