IFR and ATC

Is there a reason for some ATC flights not to have the full ATC approach instructions?I setup all flights the same way, with IFR as the default, but do not always get the instructed approach. I am going to major airports, and keeping all ATC communications active, and replying when asked.I accept that if I miss a communication that I am kicked off ATC and have to rejoin the world, so I probably have lost that flight plan. Apart from that I can see nothing else I am doing different between flights, and have even managed to work out Garmin for approaches and all that. However, it seems to be a random thing, sometimes all detailed approches are instructed, other times its just the usual find airpoet from list and do it yourself!Also, what is the minimum for an airfied to close down? I have approached major airports, with IFR flight logged, and been waved off as airport is IFR closed. I 'fly' the 737(With bruce Benaway's great panel) all the time and love the full ATC stuff when it happens!Any ideas happily received.RonaldGalsgow