filler gone soft when painting

[Paulmuzza]

I had some cracks on a painted plaster wall, I raked out the cracks and filled cracks using Wickes filler.
I left the filler several days rubbed it down and cleaned away dust residue with a dry cloth.
I mixed up some watery Dulux ceiling and wall matt paint and rolled it over the repair, for some reason the filler became soft and washed the filler away. I have tried with neat paint after stirring.
Have I done something wrong?
Could anyone please help?

[Puma]

You need to take the filler back probably. Was the filler still wet?

The very very light one strike fillers can dissolve in water sometimes, but never had a problem rolling over them.