If the model didn't accelerate on takeoff worth spit, the engine needs more. More what? We'll get to that.
If the model didn't climb worth spit, it needs more.
If the model was too fast at idle (throttle (not trim) closed) to land, it needs less.
If you're just beginning this hobby, this is enough to deal with.

When the model "needs more", that could be more thrust or more fan area or more pitch. Most times, what you need is more thrust and you get that a couple of ways. On takeoff or climb, there are two ways to get more. Very often, putting a larger diameter prop on the engine will get more. It gives you more fan area, and that's very often the key to get more thrust. If the engine can turn the prop that is. That's where you go back to your ground/tach test. Don't assume that if you go up in diameter that you'll have to also change the pitch too. That's not always the case. So slap a longer prop on and run the testing from the start. From the start. Run the engine to set the needle and then tach the sucker. The engine mfg will have given a range of props to try. If your prop is in or near that range, it's worth testing. Get near the optimum rpm. If the new prop won't do it, try another one.

If the engine needs more and you longer prop doesn't give it, you might need a prop that long but with a little less pitch. Sometimes, less gives more. Less pitch might give more pull.

If the test prop wouldn't fly your airplane at idle slowly enough for a safe landing you need less pitch. If you simply slap the same diameter prop on for the next test and it's only got less pitch, chances are good the engine will be at risk sometime down the road. It will be fairly easy to set it to run at too fast an rpm. When you need less speed, it's a good idea to put a less pitch prop on, but one with more diameter. Hey, you'll be getting more pull for free.