Thanks for all the replies.
It is just a simple Fobus; no retaining clips. It holds by friction. It has no instructions (just put the gun in and pull it out already!)
I have had the gun sitting in the holster for a week now so we'll see if that helps. There is no obvious way to loosen the retention tension so I guess I will have to work it in.
Also I will try tightening the belt (it is a 1 1/2" wide by 1/8" thick sturdy leather belt) as I was wearing it like I do without a holster.
Then if that doesn't help I will try the silicone spray. Hopefully that doesn't make the gun slippery causing me to drop it.
Thanks again