Note that citric acid works a bit more nuanced than many other descalers: it acts as a chelating agent at high concentrations (2x the Ca2+ concentration) and is more effective at removing scale because of this effect, but at lower concentrations the effect might actually be reversed because it can form solid calcium citrate, which has a very low solubility in water.
If you are using citric acid based descaler you should make sure that you are always using enough of it to avoid the formation of calcium citrate.