If the husband works hard, but does not make enough is that grounds for an Islamic divorce?

No, that's not grounds for an Islamic divorce because you married a man knowing his ability to earn.

If he is willfully not working and is not able to support you that is different than the fact that he in the job that he works and in the market that he lives in. That he is unable to make what you think is enough for you.

So if you think you deserve more, then you should have married a man that had better earning potential.

But quantifying the right of a man to be your husband or not simply because of that can be shallow.