Adjusting the set isn't too difficult. I use a simple tool made from a beech ice cream stick cut across at a sharp angle to push the tongue up or down. To get an idea of the right amount of set, look at a reed with a similar pitch that plays well. Generally there wants to be a very small gap between the bottom of the tongue and the top of the frame; the higher the reed the smaller the gap. If it's too wide, the reed will sound breathy and struggle to start at low pressure; if it's too narrow, the reed will 'choke' at high pressure. Try to bend it a tiny bit at a time; better to take a dozen attempts to get a feel for how much force is required than to bend it way too far on the first attempt.
Adjusting the set might not fix the problem, but it is worth a try anyway. The second most likely reason for slow/inefficient reeds, is that they have too wide clearance between the tongue and the frame. It is fixable by replacing the tongue, but that requires more specialised skills. A third possibility is if the valve on the other reed in the chamber isn't closing properly, that can cause a problem by allowing air to bypass the reed.
Are the problem buttons connected to very short levers, by any chance? A common problem with non-riveted Lachenal actions is when you push the button down the whole lever depresses and doesn't pivot properly, with the pad opening sluggishly and the pivot making a clicking noise. The problem tends to be worst on the shortest levers. Fiddling with the spring and making sure the cross-hole bush isn't too tight might help.
P.S. increasing the set tends to flatten the pitch of the reed by a fraction of a cent and vice versa.