Sigma 150-500 F5-6.3 focus freezing issues

[nikon.norm]

Hello, I've searched the forum but can't find any answers to a problem that I'm encountering and wonder if there are any other Sigma 150-500 5.6-6.3 DG OS HSM lens users experiencing the same focus freezing issue.
I bought the lens as a used item from a large reputable company 3 weeks ago and am using it in AF-C 3D tracking mode on my D7000. The issue is that it doesn't always focus quickly and sometimes the focus freezes and will only released if the focal length is reduced and the lens is focused on something close by and then tracked onto the subject further away. Available light is not the problem and when the lens has acquired focus it's excellent and the images are tack sharp but it's making birds-in-flight and sports photography very frustrating. I've tried the lens in the other dynamic modes (still in AF-C) but to no avail, it still occasionally freezes. Have I got a duff lens or is there a fix available for it even if it means sending it back to Sigma for calibration?
Any help, comments or advise will be appreciated.

Regards

NN.

 

[johnnypanic]

Have you checked, to ensure, that focus switch and os switches are not displaced, i.e. between the selectable settings?

Given that you only have it 3 weeks, I assume you would have some limited warranty, if it was a large reputable company you purchased from. You may be well advised to contact them.

 

[nikon.norm]

Thank you for your response Johnnypanic. I've checked all the switch positions were correct and that there were no glaringly obvious mistakes. I'll have another play around with it this weekend and see what develops =- might be finger trouble.
I'm very pleased with the results the lens gives, this is an example -I don't think it's not going to get much sharper than this:
March 25 2017_DSC747018

 

[green]

There was a recall on a batch of these lenses. I can't remember the specifics but it was something to do with auto focus. Not saying this is the issue but it might be worth checking. It was an easy firmware fix IIRC.

 

[nikon.norm]

Thank you Ben I'll check it out on Tuesday with the retailer who I bought it from. I've been playing around with it and have discovered that if the focus freezes I can over-ride the auto focus by rotating the manual focus ring to get it almost in focus and then let the auto-focus kick in to finish of the operation.
I've been using the lens for bird photography and trying to pick up and focus on a very small object some distance away is expecting a lot from any lens but I've never had a lens freeze with the regularity that this one does.

 

[green]

No worries. If I remember rightly there was a thread on dpreview about it a few years ago. Hopefully that's all it is.