- You have not memorized the tab. (mental)
- You are not TOTALLY clear on the fingering of the tab. (mental/physical but mostly mental)
- The song is physically challenging, your hands can't move into position fast enough. (physical)
- You are not able to play perfectly in time with the beat. (mental)
- You are not practicing effectively, wasting time doing things that doesn't bring results. (mental)
You don't need somebody to show you where to put your fingers. That is the kind of stuff that you learn when you are a complete beginner, in the first few weeks of learning guitar. Once you've been playing for a while, you will need more than that. You should be able to learn any song on your own if you have the tab in front of you, without any help at all. What you need is a set of strategies to overcome your mental and physical challenges.
You Have Not Memorized the TabYou need to learn a song measure by measure. Usually that means about 4-8 notes at a time. Memorize the first measure by playing it over and over again, without looking at the tab. Then memorize the second measure. Then play the first and second measure together. Continue doing this until you have memorized the entire tab, and can play slowly through the notes without looking at the tab.
You Are Not Totally Clear on the FingeringMost of the time it is obvious what fingers to use to play the notes. Sometimes it is not. Play the tab how it is most comfortable for you, but always play it with the exact same fingers every single time!
If you are playing it one way the first time around, and then a different way the next time around, it will not work. You will make mistakes.
The Song Is Physically ChallengingYour hands need to stretch in order to make complex chord shapes. You need to train your hands to play at fast speeds. You need to be able to switch from one guitar technique to the next seamlessly without sounding sloppy. In the beginning, you will only be able to play a song at a slow tempo, way slower than the song. It will also not sound exactly like the song until you continue to play it over and over again for several weeks (or months).
You won't be able to play along with the song unless you slow it down, maybe to half tempo. You can't expect perfection when learning guitar. You are trying to copy a professional player who has been playing for possibly decades.
You Are Not Able to Play in TimeThis is a completely mental problem which needs to be practiced in isolation. You will not improve your timing very much if the only thing you do is try to play along with the song or play along with a click track. You need to go much more in depth to solve this problem.
You Are Not Practicing EffectivelySome guitar players practice for hours and don't make much progress. Other players will practice for one hour and make a LOT of progress! What is the difference? The guitar player who doesn't make progress will question his ability to learn how to play. He may say things like "I just don't have natural talent." That's not true. The other guitar player is focusing his time on identifying and improving his weaknesses, practicing a wide range of musical skills, with effective practicing strategies that bring fast results.
Perhaps you make a mistake while practicing a song... I've seen guitar players do two things when this happens: they either ignore the mistake and just keep going, or they stop playing and go back to the beginning again. Both are totally wrong approaches to this, and you won't improve if you do this.
What you need to do is focus on the 4 to 8 notes where the mistake occurs, and practice that in isolation. Obviously, if the song is very physically challenging, it will take a LONG time before you can play it.