No, it's not. It just takes time. If you don't regularly speak the language you'll get rusty. That's perfectly normal. It'll even happen to your native language if you don't speak it regularly.
Also speaking a language fluently doesn't mean you're speaking it on a perfect native level without any accent.
Linguistic doors shut on you as you get older. Tonal languages like Mandarin become near-impossible for monotone English speakers to get right, which directly affects fluency. No amount of practice and lessons will make up for that missing neuroplasticity.
No, it's not. It just takes time. If you don't regularly speak the language you'll get rusty. That's perfectly normal. It'll even happen to your native language if you don't speak it regularly.
Also speaking a language fluently doesn't mean you're speaking it on a perfect native level without any accent.
Linguistic doors shut on you as you get older. Tonal languages like Mandarin become near-impossible for monotone English speakers to get right, which directly affects fluency. No amount of practice and lessons will make up for that missing neuroplasticity.