As mentioned , you can use envelopes,,,,
OR....
You can process audio. Every now and then I will either play a guitar lick or sing a line in a song and one or more notes will be correct but for one reason or another...lower in volume than they should have been.... maybe I hesitated, or the pick didn't hit the string hard enough....
I simply scroll that track to the top of the track view so it sets directly under the time line.....then I zoom in so I can easily see the low level notes. Their wave form is generally smaller.....I click on that track to select it. Next... I click on the TIME LINE and drag the mouse across the low level note area. The timeline goes gray in the selected area and the track also reflects that new selection area. I check to see if the notes I need are in that area....
Next, I use the Process Audio> Gain> and select the amount of gain I think I'll need.... generally I start with +2db gain. I click on APPLY and then set the start marker before that point and let it play through. If it worked I go on... if it's still too low I do it again. As much as is needed to get it up to the level around it.
Using this works well for me and lets me use envelopes for other things. You can also use this same thing BACKWARDS.... to lower levels.... say a note is too loud or you have a noise in the track from a mic..... simply use the reduce gain and apply it or select MUTE to hide a noise totally.