As others have said, you are being matched against people that are considered worse than you (where you are considered 'Favored'). That comparison is done by battle.net's MMR rating. It's very hard to gain points if you are always favored.
If your MMR rating is stable, then you should usually be playing 'Even Match'es, in which case your score will increase as you win, even with a 50% win ratio.
If you win several matches against opponents that are favored, then battle.net starts to pair you more and more with favored opponents, and if you continue to win, this is when your score can shoot through the roof. Winning against favored opponents brings in something like 20-35 points per game.
What you are seeing appears to be the opposite. If you lose several matches where you are considered favored, then battle.net will pair you more often with opponents that are worse than you (i.e. have a lower MMR rating).
If your MMR rating is stable, in other words you tend to win when you are favored, lose when your opponent is favored, and 50% win ratio on even matches, then your score should improve.
Also remember that as you improve, so does everyone else. So if you are improving slower than your opponents, your score will not improve as fast.
This score is not a measurement of your skill or improvement. There are a lot of metrics in sc2, and I haven't found one yet that can really measure your skill level. I am pretty sure this is intentional, because Blizzard wants sc2 to be a game without a grueling level-up system which encourages players to punish themselves against game mechanics in order to "win". Instead they are encouraging players to have fun and be creative. This is one reason they removed the "losses" metric.