The chart above is from the Morning Consult Poll. It shows the numbers for Biden and Trump before the Democratic Convention (July) and after it (August). Roughly 2,000 registered voters were queried each time, with a 2 point margin of error.
The polls show that Joe Biden didn't get much of a convention "bump". He was leading by 8 points before the convention, and is leading by 9 points now. In comparison, Hillary Clinton got a 3 point bump in 2016.
However, Biden has a much larger lead than Clinton had in 2016.
This does not surprise me. This is a different election. It's not really a choice between candidates, but just a referendum on Donald Trump. Do the voters want to give him another four years, or kick him out of the White House? Frankly, I think nearly all voters have already made up their minds about that, and I doubt the numbers will change in the next couple of months. I don't expect Trump to get a convention bump either.