Trading in spread betting - List of stocks allowed

[Varma]

Hi,

I am practicing certain strategies in demo account. I have created few screeners but when i tried to buy or short sell different stocks shortlisted based on screener conditions, i am getting different kinds of errors. Some stocks you can buy but can't short-sell and so on. I searched in the forums and i understand different restrictions such as market cap should be so and so. Are these rules defined somewhere?  for e.g. one forum reply says market cap should be above 10 billion otherwise you will get second message in below list. However even i try to short sell a stock that has £30 billion market cap, i get the same error. 

1. IG is not accepting opening trades on this market, you may only place an order to close.

2. Market you have requested is not longer available online

Any suggestions on the above will be greatly appreciated. Thank you.