Chess.com uses the glicko rating system. The glicko rating system has a second statistic for each player, called the RD (which stands for rating deviation). Basically, it is a measure of how accurate a rating is. The lower the RD is, the more accurate the rating is supposed to be.
The way that is affecting you is this: when you first start out, you have a very high RD (350, I think). In the calculations, a high RD means your rating will change a lot with each win/loss. As you play games, your RD decreases. A lower RD means your rating will change a smaller amount with each win/loss. So, as you continue playing, your RD will get smaller until it settles somewhere (depending on the frequency you play since it goes up gradually when you don't play), usually around 30 to 60 or so.
If you stop playing for a long period, your RD will start to rise again, until either you play again, or it reaches 350 again.
The title may be misleading sorry if it is. My problem here is being stuck at 690-710 in 15 min games . I only get +4 or +7 even if I beat someone 100 higher when I remember gaining much more in the past . How can I get it to go up higher? thanks for any help. I understand this might be a bad question.