One possibility is that the AP is set to operate on a 5GHz channel (Channel 36+).
If that's the case, only devices that support operation on channels in both the 2.4GHz and 5GHz bands will see the AP. The iPad is capable of seeing AP's configured for 5GHz operation so that's one data point that matches what you see.
Of course this theory only makes sense if the AP is capable of being configured to operate on both the 2.4GHz and 5GHz bands. If it is one of those, set it up to work on a 2.4GHz channel (channels 1-11 but best to use channels 1, 6 or 11). All Wi-Fi devices should see it then.
Background: Wi-Fi operates on two radio bands - one at 2.4GHz and the other at 5GHz. All devices support operation in the 2.4GHz band. Higher end devices will support both.