The phrase "we do not mean to be burdensome" is very close to the "sorry to bug you" phrase, yet has more of a formal feel. It also indicates that your intention is not to be a bother. Examples: We do not mean to be burdensome, but we are still having issues with x. and More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.