Firstly check the condition of the beam with a screwdriver or 1mm drill bit. If the screwdriver can be driven into the timber, or the drill offers no resistance, it's knackered.
Assuming it's still solid, then remember that brush on treatment does not really penetrate into the timber. The joists should be held away from the wall or on joist hangers (so that moisture does not transfer), and it may be that water is being held between the two sisted joists, work out if/where moisture is being trapped and eliminate it.
Possibly the timber is just inadequately treated, in which case just go nuts with any treatment, but remember it's not the same as pre-pressure treatment and will only prolong their life a bit.