A part from going into which one is my favorite product for descaling
which I will not since I am a vendor and don't want to be wrongly interpreted

the main issue I found with many customers is not necessarily the product they use
but HOW it gets used
example
I heard of customers dumping a powder solution in cold water and then have the machine pump do the job, well powder (granulate) will just sink in the reservoir and not dissolve (not to mention that could also damage the pump)
I suggest in this instance to
mix the powder in boiling hot water, mix until dissolved,
then let it cool down before entering a pump (or mix only half of the desired with hot then add cold
and so on). On a lever machine, cooling down is not a concern.

also the length of time in which the solution sits inside a boiler
most people will let it sit for half hour or less,
a part for the few aluminum boilers (in which solution should not sit in long)
any other could comfortably stay in the boiler for quite a bit longer
if anything after a while it will become inactive
also
IF the machine is still in working order and can be heat up safety let the heat of the machine help you doing the work

in many machines (in this case a lever machine) opening a knob and looking inside will be your best assurance that you are doing or not doing the job right