The IP binding (reservation) to the device MAC address is the important part to look at. I presume the addresses are set up this way at the router to ensure each device receives the same IP address via DHCP each time it powers up. If so, these are actually address reservations from a DHCP pool and not static IPs, though they mimic static address behavior in that they are not supposed to change based on the MAC binding.
That said, the method can present problems as previously described when APs are involved prior to the intended device address bindings behind them.
The next time you experience the problem but can still ping the address of the device in question, double check the MAC address relationship to the IP address for the correct binding as configured in the router.
Use the arp -a command at a command prompt for a list of your network's IP addresses and their corresponding hardware addresses.
C:\WINDOWS\system32>arp -a
Interface: 192.168.1.5 --- 0x4
Internet Address Physical Address Type
192.168.1.1 dc-ef-09-e5-6a-b1 dynamic
192.168.1.2 a0-63-91-3b-f7-b6 dynamic
192.168.1.4 10-da-43-b6-b1-c6 dynamic
You should see every active address on your network listed.
Compare the results with the router's designated dhcp reservations. The only place they may not line up is behind access points in bridge mode which will likely display the MAC address of the AP for anything behind it regardless of the device's IP address. If this is encountered, it may well be contributing to the problem you are experiencing..., at least with the AP in question. The solution may be as simple as switching it from 'bridge' to 'access point' if such a selection exists in the AP management interface.