Sketch Settings Automatic Solve 'Sticky CheckBox'

A little curious; just spent 30min trying to turn on/off automatic solve in a sketch of imported lines from a dwg file. The option seemed to be grayed out and not available; the turn on was requested by SW in order to dynamically solve a block drag in a layout sketch.

I went through multiple of possible combinations but option box did not come unstuck. (even tried old windows fix) Saved part, got out of SolidWorks, got back in, reopened part, opened sketch... same problem.  Then tried a different tack opened new part setup sketch and option was available to switch on. Following this I got back into old part that I was having difficulty with and all seems to now work??

I may be missing something obvious here - but not sure. Any feedback here would be appreciated.

Recently installed SW2010 current SP and using Windows7 64Bit.