test your valves by blowing into 1 end, if it does blow thru then suck on same end & place your tongue over the hole.
If it doesnt blow thru turn it around & repeat the above.
If the check valve is working it will stick to your tongue indefinitely.
if it comes loose right away then there is debris in the seat.
to clean it fill 1ft or more of airline tubing with water & blow it thru valve (you can used your air pump for this)in the proper direction,then suck again.repeat until it holds..lol (catch water in a white cup for inspection?)
the debris comes from the diaphragm in the air pump,minute bits of rubber,or bits from pulling hose on/off.
this is why it is important to check valve operation on a regular basis as described above to prevent flooding.
Major reason for diaphragm releasing particles is it is pumping against to much resistance.(usually not enough total area for air to pass thru the stone)
to overcome that if you use a air valve manifold then you would open 1 unused port to the room by a small amount while still getting desired bubbles from your stone.
so
Airpump....manifold..valve 1..check valve...airstone/etc
.......................^....valve 2...open air ((open a impossibly small amount)
.......................^....valve 3 or more? closed or to air stones etc THRU another check valve
Also Never use a manifold to restrict flow to the stones/display by closing the valves WITHOUT allowing a path for the air to escape. IE extra valve open to the air,leave valve to stones/display fully open & if you have to many bubbles then open the Free to air valve more.
check valves are ALWAYS the last item before entering the tank,if you are not using a manifold having the check valve as far as practical from the pump can help for the particles have to travel farther uphill in the tubing so gravity is your friend there.(adding an extra 10ft of hose can help for it gives the particle time to slow & stick to the inside of the air line.)
also you need to add more stones/display to the line so the pump runs freely.
to know this bubbles should stop immediatly upon unplugging pump not Peter out over 1-2 seconds.
when tuning the open to air port you will hear the differance in the sound your air pump makes as it is no longer trying to push so hard, takes some tweeking but worth it in the end.
Damit I got long winded there LOL GL