A trademark application is a legal proceeding. The law is applied to the specific facts in your case by a trademark examiner. Given that both the law and the trademark examiner aren’t perfect and because there is a lot of “grey area” in trademark law, sometimes your trademark won’t be registered.
Your trademark attorney can’t guarantee an outcome. They will give you their honest opinion about the chances that your mark will register, based on their experiences in working on many other similar cases.
At best, a trademark attorney can only give you a sense of how likely it is your trademark will register. However, research has shown businesses that work with trademark attorneys are far more likely to have their trademark register than those that attempt the trademark process on their own. See this article: https://legally.co/why-trademark-attorneys-matter