I had a similar issue on my 2002 190P. I had to spray a bit of WD40
on the sides of the slides attached to the tray, and work it in and out until
it freed up a bit. One of the battery cables was also causing it the stop a bit
short. I had to reposition the cable on the terminal slightly, so it didn't catch
on the roof of the compartment above the battery.
Another possibility may be a bit of road debris or rust caught in the slides
getting stuck when you try to pull the tray out. You might have to use a
bit of force to free it up.
Have a look at this thread. There's a pic of my aux battery tray in the
outwards position so you can judge how much farther you can pull it.
When it's working, it should come far enough out that you can see behind the
back of your battery, to allow you to lift it up out of the tray.
It's also a tight fit for height for a GR27 12V. I've shorted the battery at least once
by scuffing the roof of the compartment when pulling the tray out, as the plus
post hit the edge of the upper part of the compartment, so when you
do get it freed up, be careful, the compartment itself will ground your positive