Subjunctive mood: 'was' usage after 'I wish' and 'if only'
Solution 1:
There's nothing wrong with the sentences you write. Some people (I believe teachers among them) consider the use of the form was after I wish, if only and if (in conditional clauses) colloquial and claim that it shouldn't be used in formal speech and writing. They claim that the "correct" form to use is were. The language, however, has its own dynamics and is used regardless of the rules imposed.