It's possible to "extend" by connecting an Express to the Time Capsule using a wired Ethernet cable connection.....which yields the best performance......or.....by connecting the Express using a wireless connection......which unfortunately will result in a loss of about half of the maximum speed on the network. With two Expresses, the loss will unfortunately be significantly more.
Are you using Ethernet or wireless to connect the Express devices to the Time Capsule?
There will be more questions, but we need to get on the right track before we can make much progress.
In the meantime......try power off the entire network.....all devices.....in any order that you want
Wait a minute
Start up the modem and let it run 2-3 minutes
Start up the next device connected to the modem, and let it run a full minute
Keep starting up devices one at a time about a minute apart until the entire network is back up
Check the network again
Any change?