There may be a couple of reasons. Firstly, the stie you're reading may not have that chapter uploaded because there have been a couple of scanlation groups that did DNA in the past but they come and go so there will be chapter gaps if those chapters aren't uploaded by a new scanlation team. Secondly, due to the very same reason, as there are different groups doing the scanlation, the group that pick up later may not start translating right when the previous group left so there will be missing chapters.

Unfortunately, there are no viable solutions to your problem here since the manga has not been fully released in English yet so to read the missing materials, you have to either read the Japanese RAW from sites that have them or buy the Japanese manga. You can also wait for the English release to catch up but be aware that it's going to take quite a while since Kodansha USA has only started publishing DNA in English since March and is only up to the 3rd volume right now.

An alternative solution is to watch the anime adaptation instead of reading the manga. The anime is a faithful adaptation of the original material so you can expect to find more or less everything there including the missing chapters you're looking for. It's also a quicker way to enjoy the series so that's something to consider as well. 

Yeah I was also surprised by the sudden skip. Sometimes other manga reading platforms have the missing chapters so you can check other sites out.