Sounds like some moisture has gotten in and the 777 has absorbed it. When that happens, you pretty much need to throw it out and buy a new can.

I've had cans for 2 or 3 years that are still okay. I follow these steps for storing powder, and it seems to work:

1. Don't store it in a moist environment- like a basement. Store it somewhere dry, or put it in a air-tight box.
2. Don't keep the lid off for any longer than you have to, then make sure it is very tight putting it back on.
3. I put 2 - 3 silica gel packets (they absorb moisture- they come in the packing of various things) in the bottle, then change the gel packets once or twice a year.

I haven't had any problems when I followed this list.