The first thing to try in situations like yours is to simply turn the router off for a minute then turn it back on.
If that doesn't help then login to your router and try experimenting with different wireless channel numbers for the 2.4Ghz band. There are only three real channels so try 1, 6, and 13.
Lastly, what's the wireless signal strenghth like in the locations those devices are situated? If they're all distant from the router then can you move things around? If however things used to be fine and nothing's been moved then it's unlikely to be that.