Typically lack of damage is due to you missing a part of your combo or just not hitting your power spike yet. Most if not all mid laners each have unqiue power spikes throughout the game dependent on levels and items (the most obvious being lvl 6 for most mids). Completing combos is also very important in ensuring you deal your max damage, playing as a zed for example shadowing into an enemy then AA+E, Q, AA would be a strong combo for you as typically that combo will also weaken them enough to proc your passive on the last AA, however missing the Q for example in your combo would in most cases let them keep enough health early on to stay above 50% making so your final AA doesn't proc your passive. Missing that one part of your combo indirectly makes you lose even more damage than the damage missed from the Q alone. This is only an example for one scenario though, with the information you provided this is the best I could think of for answering your question until your provide better examples (situations, time, champion). You're welcome to add me in game if you want to play a game and show me what it is you're referring to?