sometimes works and sometimes not
Can you please describe what sometimes not means; do you get an error message or white screen of death, or something else? What is the data-error attribute for; do you have some client-side validation also? What does the ContactUSRequest FormRequest class look like?
Aside, your controller is a mess; you can get the $rerquest->validated() data from the FormRequest class and save your self multiple lines of unnecessary code? Also, why do you need a try/catch here; what are you expecting will go wrong?
I think your method can work just as well like this:
public function sendContact(ContactUSRequest $request)
{
$contact = ContactUS::create($request->validated()); // set fillable/guarded on Model
Mail::to('[email protected]')->send(new ContactMail($request->validated()));
return back()->with(['success'=>trans('front/contactus.Your message has been sent successfully')]);
}