I am having difficulty connecting wirelessly to my Linux computer with my Android mobile device. I have disabled the internal firewall of my computer and opened up access to it with port 80 on my router firewall setting. Sometimes, I am able to access my computer, sometimes, I am not able to access it. Usually, restarting the wireless connection on my mobile device would enable me to access my computer. But this time, I tried numerous times without success although I can still access the internet wirelessly through both devices. What could be causing such a problem?
If you're trying to access port 80, I assume you are meant to have a web server running off your Linux box and are trying to access it through the Android device's Web Browser.
First things first, check that something is indeed listening in on your port 80. A simple test for that is to open a browser on your linux machine and go to http://localhost:80. At this point, the browser will either successfully connect to your local server or state that it was unable to do so (message differs from browser to browser but they will go from "Oops! Google Chrome could not connect to localhost" for Chrome to "Unable to connect" for Firefox).
If you are able to connect via browser, then there might be an issue related with your Android's connectivity to the machine itself. To quickly test that out, try installing Traceroute on your Android device (only requires full network access permission which is mandatory for its goal) and perform a traceroute through it to your Linux Box's IP (start the app, put the IP on the text box and press Trace). If, assuming from your original post, you are sitting in the same network as your machine, the android device successfully connects to your linux box if you have an entry listing the IP you provided it with and several average times in a fashion similar to the image found here
If, however, only star signs are displayed (*), then the Android device most likely cannot reach the linux machine. I would advise you:
If, on the other hand, you are unable to connect through your linux box's browser to your local server, then most likely the server is not running.
Most likely, whatever service is expected to be running is executed at OS boot time (easy fix is to restart your linux PC) but it is possible that, if worked before (even if only from time to time) and now does not, some kind of update or configuration change was performed that made it unstable to run. Did you have some system update before this started to occurr?