I have had the same problem on this and another forum I use. I have found that if I click the switch editor to source mode icon, the one that appears far left and looks like A/A, the one with out the red cross, has sorted it for me. Not sure I there is a setting I need to click on in my settings page for the forum, maybe to change it to wysiwyg or something? I hope this helps for the time being.
The solution I have just found that works for me,
From the top of the page click on settings,
Scroll down to 'my settings'
Under 'my account' click on 'general settings'
Scroll down to 'miscellaneous' and instead of 'enhanced interface - full wysiwyg' select 'standard editor - extra formatting controls'
Scroll down to bottom of page and click 'save'