Same issue experienced on VS 2017 15.9.9. In your ‘Solution Explorer’ in the top right of VS, right click on on your main project (under the folder Games). From the context menu that comes up select ‘ReScan Solution’. This solved the problem for me on this occasion. Intellisense seems a bit flakey at times and sometimes I’ve found the best solution is to just come out of the Unreal Engine and VS and just go back in again.