inline frog system Inline Frog cartridge holder stuck. Cannot get it out to change cartridges.

[Zack01]

I cannot remove the inline Frog cartridge holder from our Caldera Kauai spa.  In the past, to change the bromine and mineral cartridges, I first unscrew the cap and set it aside.  I then press down and turn anti-clockwise the cartridge holder and it releases and comes out.  I can then remove the old cartridges and replace them with new cartridges.  I then put the cartridge holder back in the spa, line up the arrows and turn clockwise to lock it in place.  I then screw the cap back on.  Today, I am still able to unscrew the cap and set it aside.  But when I press down on the cartridge holder nothing happens.  It is stuck.  How can I get it unstuck?  It has worked fine for 5 years.  This is the first time I have not been able to remove the cartridge holder.  The jets are off when I do this activity.