To play at any level you have to have something you can use to win points at that level. Typically this means a big forehand or backhand, tricky serves, a strong counter-hitting or blocking game, or perhaps defensive skills such as chopping. These are a player's point-winning skills, which might be called his primary skills.
But there are also secondary skills which set up the point-winning skills. Having a big forehand loop, for example, doesn't always help if you don't have ways to set it up via these secondary skills, such as a good serve, a good receive, a spinny opening loop, a good backhand, or just fast footwork to get the player into position in a rally for the big loop.
Serves can be both a primary point-winning skill or a secondary set-up one. Players with big breaking spin serves (often long) often use them as a point-winning skill, either winning the point outright or setting up such a weak return that it essentially wins the point outright. Others have simpler-looking serves that are usually short and low, with varied spin (often backspin or no-spin that looks like backspin), that force defensive returns that might not win the point directly, but consistently set up a primary attack, usually a strong opening loop. (Receivers will generally make fewer outright mistakes against such short serves that don't have the big spinny break of a deeper serve, but because the serves are short and because they aren't always sure of the spin at first they tend to receive passively.)
Often a player seems to have a primary point-winning skill when in fact his strength is a secondary skill that sets up a point-winning one - and the point-winning one simply seems strong because he gets so many weak returns to use it against. But against stronger players, whose returns aren't as weak, the primary skill might not be so strong. These players can improve dramatically by improving the point-winning skill. Similarly, players with big point-winning skills might fall apart against stronger players who don't give them the big point-winning shot so easily. These players can improve dramatically by improving their secondary set-up skills.
Developing secondary set-up skills have a huge secondary advantage. They set the player up to attack over and over, resulting in lots and lots of attacking practice, i.e. point-winning skills, and so those primary point-winning skills improve dramatically. That's why developing good set-up serves, for example, will lead to lots of attack practice and thereby a much improved attack, leading to a higher level of play.
Examine your game and decide what your strengths are. If they are primary point-winning skills, then perhaps focus on developing secondary skills to set up these point-winning skills. If they are secondary set-up skills, then perhaps focus on developing primary skills to win the point. Often players get stuck at a level because they are missing one of these types of skills.