If your brood is “very full & healthy”, you don’t have to worry about a few nettles, I think you meant beetles.
As long as the colony has good brood frames with mostly worker comb & the bees are using most of the frames for raising brood, it’s just a matter of time before the population builds sufficiently for excess bees to venture up into the honey super. You may have put the super on too early, which makes waiting for the bees to occupy it seem like a long time.
@JeffH answered your first question, so I will deal with the second part. Supering at that point in the season is a little late in most climates for nectar flow. The main flow is over by mid-summer, unless you are in a tropical or subtropical area. That is the simple answer, however, there may be a lot more reasons, and you can use the search tool at the upper right to find hundreds of posts dealing with this. I will get you started with one I found:
Hi Dawn, I answered the second part in a roundabout way in saying that Mick could have put the super on too early. Eight weeks ago takes us back to the last month of summer. I would have thought that if the colony was ready for a super 8 weeks ago, the bees would have ventured into the super in the early stages. This is where local knowledge would be handy for Mick…
@Mick1975, do you have a local mentor? or have you joined a bee group? Were other colonies in your area bringing in honey during the last 8 weeks?
Thank you for the information, yesterday I brushed the flow frames with bees wax in an attempt to encourage the bees. Early stages yet but it seem to have had an immediate effect with a huge amount of bees now visible in the inspection window. This has never happened . Although now a little late they might be heading in the right direction . Thanks again for your help