[user]Misterwhang[/user], there was a fantastic post by Void once listing all the ways to break 'Kendama block', I can't find it right now but hopefully he will come across this post and give his expert opinion.
From my point of view there are lots of things you can do to remedy the feeling that you're not getting anywhere - every time I feel like this I spend some time doing the following:
Pay more attention to resets, untangling the string and starting 'properly'. This applies more to single tricks than anything else.
Try to invert everything you can, grips and spin directions: if you can do Lighthouse and Jumping Stick, can you do a Lighthouse with your hand upturned? Penguin? Can you manage Falling Down or Build The Lighthouse in the opposite direction? Can you flip to Lighthouse from: - Swing In, even Bird?
Break up your Around Japan/the World with some Whirlwind/Tornado type tricks: try to flip the ken every which way between Big Cup landings - now Small Cup in sara grip - now forward and reverse between Base Cup - bend your knees and get some huge height in there at first, and try to work through the Around with a different flip in between each landing.
Change the time and place of practise - this might sound ridiculous, but I often have days where it is impossible to land anything much in my room, and on getting out to an inspiring spot, I start breaking out of old patterns and can see new ways of linking tricks together. If you usually practise in the evening, get up an hour earlier, 'dama in the morning, and vice versa.
Eat some mixed nuts or some dried fruit, try again.
Try to get a modification into those 'easier' tricks of yours - start at Spike and go for an Earth-Turned Bird, or try for a Swing-Bird. Tap your Lighthouse and see how it spins, just experiment with how you might catch it. Try a Corkscrew Jumping Stick - put your free finger on the Big Cup on take-off to get it to spin sideways as well as towards you.
I am pretty convinced that messing around with these things improves your general kendama control greatly, which is half way to opening up a whole new avenue of tricks, and presents you with the opportunity to think outside of trick ladders.
Don't give up!