2) Communication doesn't improve. It's normal to have communication issues especially in the beginning, but these should be constantly improving with time.
3) Trust issues remain. Again, it's normal not to fully be able to trust someone at the start, as trust is not just a given, but it is earned. However, over time all the worries and trust issues you have should be completely gone if your relationship is going to last.
4) It's boring. I'm not saying relationships last by going out every single weekend and doing activities. I am, however, saying that even when you're not doing anything, your partner should never bore you. If you can have fun and laugh with nothing, that's how a relationship lasts.
5) It's stagnating. Every relationship moves at a different pace, but the important thing is that it is moving and evolving, or there is at least talk about future steps. If you're in your 20s, your relationship shouldn't feel like a high school one...
6) Families don't approve. I'm sure there are a lot of exceptions to this, as everyone has very different relationships with their families. However, for some, whether the relationship will last or not can definitely be affected by family approval.
7) Living together is just not a good time. If you've taken that next step, you may unfortunately get a huge reality check on what your partner is REALLY like, and it may not be so pretty. Some people just aren't a great match, and sometimes it takes living together to really see that.
8) The chemistry disappears with time. Of course, the initial chemistry and "butterflies" change over time, however they never disappear. Those feelings of infatuation should evolve into feelings of love, making your connection even stronger. If you're feeling less connected to your partner with time, it's not really the best sign.
9) You're having doubts or feel your partner is. Don't ignore your gut, our instincts are definitely more accurate than we think.
10) You can't work as a team. As we grow up, it's only normal that things change. Our passions and goals change. Our jobs and responsibilities. Our priorities and schedules. Really the list can go on forever, because life always throws sh*t our way. Point is, the longer you are together, the more changes and challenges you will experience together, whether they occur with one or both of you. This will often require you and/or your partner to adjust and adapt to these challenges, and although some may be harder than others, the important thing to notice is whether you are able to function as a team, and get through anything life throws at you together. If life challenges only cause even more stress on your relationship, and you are not able to be there for each other and work as a team, I'd say that's a pretty big sign your relationship may not be the one.