To the OP’s issue:
As pointed out by LinkRunner, initial config is via bluetooth direct to the Sense unit. If you have two of them close by, it is possible that your phone isn’t sure which Sense to say hello to when trying for a direct connection for initial configuration.
But once configured, it should rely on WiFi and then the Sense talks to “the cloud” and your phone/app talks to the cloud.
If your app is losing connection to the Sense after config its either a network connectivity issue with your phone to the internet (unlikely) or a issue for the Sense to the the internet.
Did you use the antenna extender to get the antennas outside of your panels? If not, I would certainly try starting there.
If they are out as others have mentioned, getting a stronger signal to your Sense(s) will probably make a big difference.
Going a bit off topic but to expand a bit on what @kyle and @rjgroves mentioned about extenders and additional AP’s.
Randy is of course correct that running an ethernet line is not always possible.
I would recommend people look at powerline “extenders” that offer integrated wireless.
Along the lines of something like - http://www.tp-link.com/us/products/details/cat-5509_TL-WPA8630-KIT.html
But I would never recommend using the Wifi Clone feature. Always create a separate network name. Unless you are using a controller of some kind, having multiple AP’s broadcasting the same network name usually ends with problems.
Over simplifying, but most extenders work by essentially taking the available bandwidth they can get from the AP they associate with and then chopping it in half. Then depending on how many stations associate with the main AP and the extended AP, your signal performance goes down even more. Most extenders don’t have any good airtime fairness algorithms built in.
The issue I run into more often than not with extenders is people saying “but my device shows great signal. I have full bars”. It may have a “great signal” to the extender, but how is the signal from the Extender to the AP its “extending” the signal from.
If you can get any stats from your main AP and your Extender, I wonder what it shows as TX/RX rates between extender to AP and then extender to Sense. Knowing that the Sense is particular about is connection to the internet, if it is getting a low data rates to the extender, it may be causing some of the issues. Especially with the number of devices you are saying connect to that extender. I’m assuming its single radio, 2.4Ghz. I would bet money that the sprinklers or weather station may have an cheaper Wifi card in it that only supports 802.11g data rates, so it takes down the performance of everything else on the extender.
Anyway, that was a really long way of saying “hey, if you are considering an extender, consider powerline”. You can still run into some of the same issues, but power line has come a long way. Range extenders, not as much.