Gas leakage is very dangerous, which sometimes can lead to fire outbreak or gas wastage. There are various problems that can lead to leakage of LPG in your cylinder. Although, the causes varies based on the types of gas cylinder.
WARNING!!!
FOR HOME SAFETY , TO PREVENT FIRE OUTBREAK INSTALL LPG(GAS) DETECTOR ALERT  TO AUTOMATICALLY RING WHEN THERE IS GAS LEAKAGE. YOU CAN GET IT HERE.
Gas cylinder with burner on top of its head (e.g. 3kg, 5kg, 6kg cylinder)
On this types of gas cylinder the gas (LPG) can leaked from the gas cylinder body, control or valve.
Gas cylinder body: this problem is normally cause by gas cylinder made from inferior material. The best solution is to replace it. You can buy original gas cylinder her or on jumia.
However, some professional metal welder can help to fix it by melting iron to cover it up. Please note that you must empty the cylinder and remove the gas valve to pour soap water inside to clean away gas residue before melting.
Faulty burner control: the gas cylinder burner sometimes may not be locking well even after you think you have put it off. To test if your burner is faulty, just tie the burner back on your gas cylinder and remove other part leaving only the control. Then, turn it off completely and pour small quantity of water on it top where gas usually passes through; If there is any leakage you will see the water forming bumble .
In some cases you don’t even need to untie the burner part, just by turning it off and testing the cap with a lighted matches or lighter; you will notice it burning in slow but not steady quantity. The best option stop it is just to change the control or complete burner totally. You can buy original here
Faulty valve: if the gas cylinder valve is weak  or not well tied, it can leads to leakage from the side as shown below.  Gas can also leak from inside the gas cylinder if the burner is been removed from the top of the cylinder.
To solve this problem  follow the step blow
Make sure you have finished the gas inside then unloose the valve using adjustable spanner or retches.
If the valves are still good that is, it is only leaking from the outer part as shown below, then just get a Teflon tape (buy here) and wrap it run the valve multiple time and retie it back to the gas cylinder very well.
In case the valve is bad (leaking from the middle or spoilt in the process of loosing ). Then, buy a new one (buy here) and use teflon tape to wrap it very well and tie it back to the cylinder.
For standard cylinder with separated gas burner
Major causes of leakage in this type of LPG setup are faulty regulator, pipe, gas stove and valve
Regulator
If the regulator is faulty, will be gas leakage from the gas valve.  This fault can be identified when you notice the leakage is from the regulator , it will be giving you a continuous sound of flow.

To solve the leakage from regulator
Change the valve seal rubber. If problem persist , change the seal rubber for the regulator. Image as shown below.

Buy new and original gas regulator here or on jumia
Pipe: pipe can be eaten up by rotten which leads to leakage. The perfect solution is to buy iron made certified LPG pipe here
Faulty valve: if the gas cylinder valve is weak  or not well tied, it can leads to leakage from the side as shown below.  Gas can also leak from inside the gas cylinder if the burner is been removed from the top of the cylinder.
To solve this problem  follow the step blow
Make sure you have finished the gas inside then unloose the valve using adjustable spanner or retches.
If the valves are still good that is, it is only leaking from the outer part as shown below, then just get a Teflon tape (buy here) and wrap it run the valve multiple time and retie it back to the gas cylinder very well.
In case the valve is bad (leaking from the middle or spoilt in the process of loosing ). Then, buy a new one (buy here) and use teflon tape to wrap it very well and tie it back to the cylinder.
WARNING!!!
FOR HOME SAFETY , TO PREVENT FIRE OUTBREAK INSTALL LPG(GAS) DETECTOR ALERT  TO AUTOMATICALLY RING WHEN THERE IS GAS LEAKAGE. YOU CAN GET IT HERE.