I see two possible issues here.
The documentation says that the maximum message length is 60 characters. Your message might be too long so it is truncated.
The documentation goes on to say
Invites (requests where the recipient has not installed the app) do not display this value.
So this means that if this is the first apprequest this user has received prior to installing the application, s/he will not see the message but only the "invitation" to start using your application.
Using the requests dialog, you will be able to "predict" whether the user has installed the application or not by using the filter parameter. Set it to app_users to only display users who have already installed - in this case you can include the (60 character limited) message parameter. If you set the filter to app_non_users, you might want to include the content of your message in the title parameter as it is always shown. The title parameter is limited to 50 characters.
FB.ui({method: 'apprequests',
title : 'Test App',
message : 'Test app message',
filters : ['app_users']
}, requestCallback);
Which ever way you decide to go, you'll want to have a relatively short message/title for your invites/requests.