Time of day, and what day it is, can have a great effect. Why you may not notice that this is a factor. Also, if they are doing less intensive things and then start a game, movie, etc., that will do it. Also phones, not always would they be accessing via WiFi. Depends on what is being done.
You may also want to try securing your network. Change the password and name if need be. You could have someone in the area that is accessing it which would only be effected when they are accessing it.
If yours is the only device having issues, you may also want to try the following (as computers tend to have more sensitive connections).
Try this....
First, make sure your laptop is 'not' connected to the modem/router via an Ethernet cable and follow the steps:
1. Go to "Control Panel".
2. Next go to "Network and Internet" and then "Network and Sharing".
3. Now click on "Manage Wireless Networks".
4. On this screen, remove all wireless connections by "right clicking" on it and select "Remove network" from the drop down menu.
5. After doing this, close the open windows and reboot the computer.
6. Once the computer is fully reloaded, scan for the "Wi-Fi networks".
7. Click on the available network that you want to access and select "connect".
8. It takes a moment, but then you will be asked to give a password for your Wi-Fi connection. Type in the password and click the "connect" button.
It should now be connected.
NOTE: If this doesn't work, you may want to reset your modem/router, and then once it is fully loaded again, repeat the above steps. Yes, issues with the modem/router can cause issues with only one device.