Take the grille off and watch it work while someone else pulls on the lever.  You'll find the place that needs some lube. 

In any case fix it now before won't open.  Also, in the interest of not getting stuck somewhere, install a pull cord on the hood release and tape it up someplace that is accessible.  If it ever sticks for good you will be very happy you gave yourself an out.

You are probably on to something with the idea that the lifting spring is losing it's strength.  My release mechanism is smooth, and the hood does unlatch. But is just lays there and doesn't jump up the ~2cm it used to. 

The EPC claims that on, my 1996, the spring is available separately (A0008870179) for a $6.  It is part of the hood side of the latch.  I haven't actually looked at how hard it is to replace.

 When I encountered this problem, I was deep into the Rockies, and just longing to open my hood to check fluids and just say "Hi" to my 617 diesel after a long day of 9000 ft. passes, and other hard work.  Added  to the "hard to open" condition, I could later not close the damn hood once opened. The latch had let go and it was just needing to be replaced. I had to tie down with a rope - very uncivilized, for a Mercedes. The solution was to buy a new latch, new cable, and to make certain all bits were aligned and taunt according to specs. This took some trial and error, but in the end, I now have not problems with hood opening. The new "cable" was much better than the old wire pull device.  

 It has become even more important now to be able to open the hood, since on our camping trips I put a pork, beef, or chicken roast on the manifold wherein meat, veggies, and other goodies are roasting while we travel to our target camp site. Not being able to open the hood means we eat trail bars...again.

Chef Tom