Not holding on Tamron 70-300mm

The lid does not hold good on my Tamron SP 70-300mm F / 4-5.6 Di VC USD.
Just give us a only very slight pressure and the lid will spring from the lens.
Too bad, because the cover itself makes a good impression.

I've looked at the lens cap by JJC, Tamron and Sigma exactly.
If you turn the lid can be seen that the locking mechanism of the JJC is the same as the outer ring, on which the thread is positioned with which the cover is put into the threaded ring of the lens.
Unlike to the cover of Tamron and Sigma, where the 'inner life' clearly rebounds - both cover very well hold the Tamron lens.

Now if you look at the lenses we see that the Tamron, the front lens has an outward curvature on which the JJC-cap is seated and can not properly lock into place.
The Tamron and Sigma-lid are not on hold and therefore very good.

At Sigma Lens keep all cover - also the JJC - very good. Reason: The strong threaded ring at Sigma and no protruding curved front lens.

See also photos.

Conclusion:
The cover makes a solid impression and keeps very well on my Sigma AF 30 mm.
Unfortunately not well he keeps on Tamron 70-300 due to the above characteristics, so a star deduction.
For the money, but always a recommendation.