Ok, so I was trying to work this out myself and I can't fix it.
The last few times I made mmf, I got lots and lots of sugar lumps, small ones, about the size of a bb. I have sifted my sugar, sometimes more than once and it doesn't seem to help. so now I am wondering if it is the temperature of my marshmellows when I am combining the two. I used to always melt my marshmellows in a double boiler but now I have started to do them in the microwave, I am not sure if for some reason that has created the change.
once I knead the stuff, for about an hour it feels like, they tend to dissolve, but I have never had to do that before.
any advice would be greatly appreciated.
Kym
are you doing everything the same way as you did when you melted them in the double boiler? are you using the mmf recipe from here?
i just made mmf just now and for the first time, it did that to me too... i've never had that problem before... 
Remedy: Start out by mixing in a small amount of PS at first (maybe 1/4 c to 1/2 c). This helps absorb moisture so that when you add more the PS won't tend to roll itself into little balls.
I always use Wal Mart brand but recently they were out, so I tried Target's brand and I did notice tiny balls wanting to form. I just kept adding a small amount at a time and avoided having them embedded in the MMF when finished.
FYI: If you overheat the marshmallows you risk your MMF getting too hard. It happened to me once. The MMF became too hard from accidentally overcooking and required less PS, so when I softened it in the microwave it became too sticky.
Quote by @%username% on %date%
%body%