The resin will be ok , just keep it in the container with water in it ,
I think the problem you have is the size of your container , your using a 25 L drum , i would say the sizes are 10" x 10" and its got to be 12" high .
If those measurments are about right then your resin cant be that deep !
My 10 L bin is 7" x 6" and 12" high .
This means the base area of your container is 100 square inches
Mine is only 42 square inches , thats half of yours !
Therefore , if you have used a quater of a bag of resin , i estimate that your resin is only 4" deep , am i right ?
My resin is at least 8" deep , so , my water is having to flow through twice the resin that yours is , change your container to the same sizes as mine and i bet it solves your problem .
Rich P @ F