Help with the rspec and the xpath

Hi Geeks, i need help with the rspec, Some time i having a problem
with locating the element with the xpath
Same element, i can able to select some time where some time i can
not. Is there a bug or i need to do some specific for that?

My way to locate xpath

browser.link(:xpath,"/html/body/div[@id=‘page’]/div[3]/
div[@id=‘right’]/div[@id=‘control-panel’]/ul/li[12]/a/img").click
browser.checkbox(:xpath,"/html/body/div[@id=‘page’]/div[3]/
div[@id=‘right’]/form[@id=‘ModulePreferenceSettingsModulesForm’]/table/
tbody/tr[3]/td[2]/div/input[@id=‘ModulePreference6Enable’]").clear

I use firefox addon Xpather that create xpath of the HTML elements.

I appreciate all reply please give guidance.