Not sure what to say. I haven't had that issue yet and I've been using the salt on my rides and runs for over a month now.

Are there any small seams or cracks in your tube? Sounds like excess moisture is getting in.

Possibly, I'll have to check

A couple of reasons that this could be happening:
• If your hands are dripping wet when you cover the hole, the moisture will drip into the salt causing clumping.
• If you leave it open in a humid environment moisture may get into the vial.
• If you touch your tongue to the opening this could lead to clumping.

How full are you filling your vials?
For races I would suggest filling them 1/2 of the way up.
For training days only fill the tube about 1/4 - 1/2 of the way up as this will give you plenty for whatever you are doing.

Continued use of the same salt in the tube means you are continually opening and closing it which can add bits of moisture, resulting in possible clumping.
Try the suggestions above and if you continue to have clumping issues you could add a small bit of rice to the tube to absorb some of the moisture.

Hope this helps.

Bet that's my problem... I filled one tube all the way and have used that for a couple 2.5 hour runs and a 3.5 hour ride. Never thought to just put a little in at a time.

Just bought this product myself and used it for the first time. Seemed to work well with the exception of when the lid of the container was open right after I wet my thumb and took some product when I hit a bump in the road and there was product all over me...

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"For those who understand, no explanation is necessary. For those who don't understand, no explanation is possible."