Just come back from my lesson and had a similar problem, however it was on a cessna 152 so i'm not sure whether you could remidy the problem the same way, but here it goes.
Increase the throttle to approx 2000rpm and then lean the mixture until you get start to get a slight drop in the rpm. hold this for maybe a 1 min and then set mixture fully rich. set power back to its original setting and try the magnitos again. As i said it worked for me but it was on a cessna 152. Another method my instructor showed me on a previous occasion was to leave the power as it is and lean the mixture until you get a drop in rpm and leave it like that. This also worked in my case.
brgds
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