This can happen and it is not unusual , a wireless router has a limited range , usually 150 feet but in addition to that is the solid walls and floors in multi-level homes.
Your best option is to add a signal repeater which will boost the signal further and into more areas.
Step one. Determining where to place the repeater
You will want to take your laptop or tablet and move around starting close to the router and moving away from it.
When you get to a point where the signal strength is 50% or lower , your getting to where the repeater will need to be placed.
Step two. Determining the remaining coverage vs. the current coverage.
If your at a place where the signal strength is low and your thinking it would be a good place for the repeater, you now want to look at the overall coverage to see if the repeater will cover the remaing space since you don't want to place the repeater only to find that it's not the optimal spot.
Read through the user manual of the repeater to find where it will state what the repeaters coverage area is.
Once you find the distance that the repeater will cover you can see if the position you have found will be within that coverage area.
Step three. You will now have to run a cat 5 cable from the router to the repeater if you purchased a repeater that requires a cable connection to it to extend the network.
If you purchased a range extender then you won't have to do that and all you will need is an AC outlet.
The Linksys RE-1000 is an example of a range extender.
Step four. You now need to take the repeater/extender and connect it to the router and open a web browser and type in the address of the device. You will need to change the SSID and WEP to match the router and the channel needs to be the same also.
Once everything is connected and running you can now test all the areas that were previously without signal to verify that the repeater is supplying sufficient signal strength now to make an internet connection.