The gas struts used by Lotus to support the Evora tailgate seem to loose their strength rather quickly.
From having to help the tailgate to be brought to the top position to not staying in the top position as all is wrong. From about half way position, the gas struts should take over and lift the cover for you after which the cover should stay in position until you close it manually.
These replacement gas struts are cheaper than the original, but do the job they should do!