FYI, there are 4 unique botanist levequests per level range.
And only two are given at any time.
If at a specific range 2 are given that you have already done, pick them anyway, complete one but don't hand in, check if any new levequests are available. If not, complete the other as well, but don't hand in. By now you should be able to get at least one more. Complete that too without handing in and you should be able to pick up the remaining one.