It is claimed that many adjectives cannot be used in a predicative position. Commonly, entire, utter, former, and potential are claimed to be of this type. Attributively, they look like this:
- The entire class helped.
- There was utter devastation.
- He's a potential client.
- He's a former client.
Personally, I have no problem using most these words in the predicative position. I'm a native English speaker, and they all seem fine to me. However, many English speakers have problems with:
- I toured the areas affected by the hurricane. The devastation was utter.
- He's so unlikely to spend any money, of all our possible future clients, I consider him the most potential.
To me at least, "the problem was major" is perfectly fine. If you asked me how anyone could find a problem with it, it likely wouldn't even occur to me that major is claimed to not be usable predicatively. But even I can't find any way to use entire predicatively.