which screws in the capsule is tight.
And always lube every 4th capsule head.
Just lighly lube it or it will gunk up the internals.
If lubing the next capsule and tightening the screw cap does not work, then you will have to replace the valve seal.
They should last a long time if you lube the capsules and don't leave the capsule in too long.
If you regurlarly leave it in for a week then this will drastically reduce the seal lifetime.
Altough the seals are dirt cheap, it is a hassle to replace them (asuming you can strip the gun apart), because you have to completely strip the gun.
This is the longest job in terms of time.
So take care of your seals or it will cost a lot in maintanence.