iPhone will not automatically connect to one particular wifi network There is a wifi in my home (run by landlord - I have no control), which is called &quot;Default&quot; and has no encryption enabled.
I can manually connect to the network from my two iPhones - but whenever an iPhone loses connection, it does not automatically connect again when in range.
What can cause the phone to not automatically connect to a know access point?
Both my computers connect and reconnect with no issues.
I have not seen this behavior with these phones with any other wifi access-points.
I have tried resetting the network settings to factory defaults on one phone, but it did not make a difference.
Both iPhones are up to date with latest iOS updates (5.1) and are not jailbroken or anything out of the ordinary.
Let me start this somewhat lengthy soft answer with a suggestion:

See if your landlord will allow you to extend his network by allowing you to add an Airport Express in your apartment. This may be the miracle solution you need to what is otherwise likely to be complex and quite possibly unsolvable problem otherwise.

I strongly suspect you're looking for a technical explanation to your problem. That answer is likely to be lengthy and beyond my expertise. It will also be largely theoretical because without some specific tests performed in your environment, the truth is unlikely to be known, only speculated upon.

The practical answer is that sometimes certain access points don't like certain wifi transceivers. Several years ago I had massive wifi problems with my the just-purchased MacBook Pro and a LinkSys wifi router that had been serving the house successfully for years—and still worked fine for all the other computers in the house, including my iMac G5 running the same version of the OS. 

I applied numerous patches and tried various*tricks* on the Mac. I installed  alternative firmwares onto the Linksys. All with no success.

We installed an Airport Extreme and all of my wifi woes went away. Instantly.

If you're looking for explanation, you'll have to look further. If you want a solution to the problem, see if you can install an AEX, enjoy full use of your device, and don't sweat the details.