I'm not familiar with this printer, but:
# "Loose" does not describe anything I can relate to.
# Placing the straight edge of a steel ruler from corner to corner might tell you if there is a dip there if that is what you are refering to.
# Knowing how you leveled the 4 corners might help with the analysis of the problem.
# You can print a flat object from corner to center (move it from the default center position) to see if it is only not sticking in the center.
# If it also doesn't stick on the corner, your Z offset may be the problem, or your Z endstop may be loose or inconsistent.