return withSuccess withErrors lost - laravel

Question

I'm using a request validation to validate my form post which works fine for a "simple" contact class but not for my posts class.
Working:
ContractController GET:
public function showForm()
{
return view('contact');
}
ContractController POST:
public function sendContactInfo(ContactRequest $request)
{
blabla
return back()
->withSuccess("Thank you");
}
not working:
postcontroller GET:
public function changepost($postid)
{
$postdetail = posts::find($postid);
blabla
return view('changepost')->with("postdetail", $postdetail);
}
postcontroller POST:
public function changepostvalues(changepostreqeust $request)
{
blabla
return back()->withSuccess("Thank you");
}
withSuccess and withError is only stored for the next request, correct?
I've also tried to use Session:flash with the same result, success and error are not in the session.
when I try Session::put all work fine. however I don't want to store the messeage permanently in the session and I also want to understand why the infromations are lost...
I believe it has something to do with the return view('changepost')->with("postdetail", $postdetail); where I need to pass also the error and success messages....
Thanks.
EDIT
Add View
#extends('master')
#section('title', 'lala')
#section('sidebar')
#parent
#endsection
#section('page-script')
<link href="{{ asset('/packages/dropzone/dropzoneaddpost.css') }}" rel="stylesheet">
#foreach($images as $image)
<meta name="postimage" content="{{$image->image_path}}" id="{{$image->id}}">
#endforeach
#stop
#section('content')
<div align="center">
<div class="addpost">
#if (Session::has('success'))
<div class="alert alert-info">{{ Session::get('success') }}</div>
#endif
<div class="row">
<div class="files" id="previews">
<div id="template" class="file-row">
<!-- This is used as the file preview template -->
<div>
<span class="preview"><img data-dz-thumbnail /></span>
</div>
<div>
<strong class="error text-danger" data-dz-errormessage></strong>
</div>
<div style="text-align: center;">
<button data-dz-remove class="btn delete mydelbutton">
<i class="glyphicon glyphicon-trash"></i>
</button>
</div>
</div>
</div>
</div>
<div class="row">
<form action="/changepost" method="post">
<div class="form-group">
<label for="txttitle">Titel:</label>
<input class="form-control" id="txttitle" type="text" name="beschreibung" id="beschreibung" maxlength="45" style="" value='{{ $postdetail->beschreibung }}'>
<label for="txtmessage">Beschreibung:</label>
<textarea id="txtmessage" class="form-control" rows="5" name="description" id="description" maxlength="500">{{$postdetail->text}}</textarea>
</div>
<input type="hidden" name="_token" value="{{ csrf_token() }}">
<input type="hidden" name="postid" value="{{ $postdetail->id }}">
<button id="savepost" class="btn mybutton start">
<i class="glyphicon glyphicon-upload"></i>
<span>Save</span>
</button>
</form>
</div>
</div>
</div>
</div>
<script src="{{ asset('/packages/dropzone/dropzone.js') }}"></script>
<script src="{{ asset('/packages/dropzone/dropzone-config.js') }}"></script>
#stop