Nominalization means that any other word (verb, adjective, adverb), which is not primarily a noun, is changed/derived into a noun/ is used in nominal function.
I think that 'an issue' is the case of nominalization since 'to issue' is a verb. I don't think this is the case with 'problem', which is already a noun, so it can' t be nominalized...
A complex noun group consists of more parts: a car - a simple n. group, my new car, a friend of mine, the man I met yesterday - complex noun groups
I hope I've helped.