I've seen others mention not being able to convert a bonsai shrub at all but my problem is a little different. I can convert it but at the same time I can't convert it.
If I click on the bonsai shrub with one sim there is no option for him to convert it but if I switch to the other sim in the household and have her to click on the bush the option is there and she can convert it without a problem.
As far as skills go they are both the same. They are both maxed in gardening and both have the green thumb reward. About the only thing different about them is on is male and one is female. But I wouldn't think that would matter.
The shrub
When he clicks on it the is no option to convert
When she clicks on it the option to convert is there.