Fly to the site as per your mission objective, then just fly around and wait until your mission objective acknowledges that you've arrived by changing to something like 'Kill all sentries - 0/9'.
The servers are laggy as **** and getting worse, so it often takes a minute or two for them to catch up.
Then, and only then, start shooting the sentries. Any you shoot before the objective changes will not be counted, but new ones will not spawn either. You only get the 8 or 9.
If you've done all this correctly and still cannot find the remainder, fly around in wider arcs and try using the scanner a bit. It can sometimes take a while, but you should be able to find them all eventually.