can't progress to Arbiter boss: no point 9 and 10

IsI'm doing tier 15, 16, 17 and 18 maps and to get all the boss skill points. I ended up with 8 points, but the last two are not dropped. Others have had the same problem, which I think is due to the fact that we did not follow a progressive order in completing the maps: personally I did t15, t16, t 18. Trying to do t17 nothing happens, and neither does repeating t 18 Has anyone encountered the same problem?