Sakura Gel Pens Dry/Plugged

My Sakura Gel Pens are not writing like they used to and they still appear to have lots of ink...and they haven't been used often so I'm pretty sure they're not out of ink. Any suggestions on getting them flowing again. Thanks!

I have a bunch of those. 2 things I've been told;
1. Store all ink pens flat. I don't know if that is why mine always work, but I do and they do.
2. Warming the pens thins the ink. You can't put it in the microwave, so I would use my heat gun ... at a distance and limited time. I don't have the problem so I haven't tried it.
In my little world, if the pen don't work, toss it and get another one.

Yes, I was going to make a suggestion along Don's with heating them up. I would use hot water in a glass and put them in for a couple of minutes.

What I usually do is:
1) Scrape with my fingernail on the 'ball' at the point of the pen. Make sure that the ball can move freely.

2) Take the pen out of its housing. Blow at the open end while trying to draw at the same time. It's a bit tricky and you will need to blow hard, but it usually solves the problem for me.
Alternatively, suck at the writing end of the pen. I find that to be less effective though.