On my yahoo account i have recently setup a new campaign and am trying to experiment with different ads (3 at the moment in each ad group). Yahoo keeps pausing 2 so only 1 is running - negating any testing possibilities. Why do they do this? Is there a way to stop this from happening again?
Have you addressed with Yahoo? If you're sure that noone else from your company, client, etc is logging in and making changes then it could be due to recent change to their TOS. The changes basically allow Yahoo to access and make changes to your account WITHOUT your prior approval. You can email them and opt out of it:
OPTIMIZATION. In the U.S. only, for those advertisers not bound by an Insertion Order, we may help you optimize your account(s). Accordingly, you expressly agree that we may also: (i) create ads, (ii) add and/or remove keywords, and/or (iii) optimize your account(s). We will notify you via email of such changes made to your account(s), and can also include a spreadsheet of such changes upon your written request. If you would like any of such changes reversed, please reply to such email within 14 days of the change(s), and we will make commercially reasonable efforts to reverse the change(s) you specifically identify. Notwithstanding the foregoing, you remain responsible for all changes made to your account(s), including all click charges incurred prior to any reversions being made. It is your responsibility to monitor your account(s) and to ensure that your account settings are consistent with your business objectives.
Another thing I forgot to mention is if you're opted into Optimize Ad Display (where ads with better performance are be rotated more frequently) then Yahoo system will eventually turn ads with poor performance off. So you may want to opt out of the ad group optimization feature. I'm generally opted out so forget about that as an potential culprit :)