When I went back to my ferric ammonium citrate to do cyanotype, I had the unpleasant surprise to find it completely solidified, really like stone (a iron one I guess).
It was stored in a warm and dry place, in a sealed container.
Do you know what happened? so I can prevent it next time...
If it is the concentrate crystals to make the sensitiser you can just dissolve it and use without issue I was told when I mentioned this to the local vendor.
Keep in airtight cool dry place...
If you read the MSDS sheet for this chemical you see that it is described as Very deliquescent. I don't know what sort of container was being used for it but it needs to be stored in a well sealed glass one. Exposure to the air should be minimized so weigh it out promptly. In addition protect it from light.