The end plate may be upside down. Turn it around so that the holes are on the bottom. In the future, never remove the end plate. Wash it in place to avoid potential for error in reassembly.
Make sure the filter papers are installed correctly with the holes at the bottom. The filters also should be flat with no folds or wrinkles. Check for tears or tiny holes in the filter papers.
Your press may not have been thoroughly cleaned after the last use. Recirculate the syrup back into the warming tank until clear. OR clean the filter press before starting again.
Syrup should be between 180o to 190oF when leaving the press. If your syrup is too hot, new sugar sand can generate during/after filtering.
A troubleshooting guide is available at the back of your Filter Press Instruction Manual.