Interpreter services are extremely helpful in locations and situations in which a person might not speak and understand the same language as everyone around them. However, having someone who speaks "the language" might not be enough in some cases. Many languages have dialects, and sometimes those dialects are hard for other speakers of the same language to understand. While there is often a standardized dialect in a language that everyone can use, occasionally you find someone whose pronunciation or use of the language still diverges. In those cases, you need someone who speaks that dialect or one close to it, and not just the standard language.
Spanish is a great example of how pronunciation of the same language can vary widely. You've got the Cuban use of /h/ instead of /s/, particularly at the ends of words; and the European Spanish /th/ sound, which doesn't exist in Spanish in the Americas. These alone aren't enough to make interpretation difficult for most people, and they're well-known enough so that interpreters and listeners can adjust expectations ahead of time. If you know that someone pronounces an /s/ like an /h/, a Spanish interpreter can tune their brain in to it and have no issues.
But if that person is speaking a regional dialect within those dialects, especially if they're speaking fast and using words that have different meanings in different Spanish dialects, then interpretation can become difficult. The interpreter could end up mistranslating a few words, especially if the person needing interpretation uses a lot of local slang.
And then you have dialects that diverge a lot and that are usually spoken very quickly, which can cause confusion when detail is needed. Chilean Spanish often gets the brunt of this confusion, along with Argentinians using the Vos form, which isn't really used anywhere else. In a measured situation that needs only basic interpretation, most Spanish interpreters will be fine. But in a stressful situation like filing a police report when upset, then you really want an interpreter who can handle the particular dialect.
And it's not just Spanish; most languages have issues like this. Think of some of the very thick regional New York accents; if you're not from there, and you've never heard them before, the first time hearing them can take some adjustment. They're both perfectly decent dialects, but the pronunciations can be difficult for first-timers to understand.
If you know you need a certain dialect, do what you can to find an interpreter who speaks that dialect. Good interpretation and translation companies will have on record who speaks which dialect, which will make your task easier.