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My dryer only heats on one cycle but that one cycle doesn't heat very well.
I am not sure about this unit. Is the above the way it is supposed to work or is it that some heat cycles are not working.
If not working which ones work and which do not.

The most common cause for having to use multiple cycles is a vent problem.

Unplug the unit and disconnect it from the vent system.
Now try a run.
If it now dries OK odds are the vent system needs checking/cleaning.
Be sure to check that the louvers on the outside of the house open correctly.

If you do not want any lint in the house.
Take a pair of pantyhose.
Put one leg into the other and then attach this to the dryer's vent.
Leave enough room so the pantyhose can balloon out like a windsock.

Both the above will let you check the temperature and the air flow.

Note: The above is not recommended for gas dryers due to carbon monoxide concerns.

See the attachment for your wiring diagram.

At the top of my list would be the timer.
Unplug the unit and one wire to the Timer Switch 2 contacts.
Then measure across the contacts with a meter.
Set it to where it does dry, the meter should read 0 ohms.
Then set it to the cycles that do not work, they also should read 0 ohms.
If not the timers is toast.

If you do not own a meter, I would suggest you purchase a one. You can get a decent digital multimeter for under $20.00. You do not need fancy though it is nice if the leads are a couple feet long.
If it saves ordering one unnecessary part it has paid for itself and you end up owning a useful tool.
Most places will not let you return electrical parts so if you order it, you own it.
A couple things to watch when measuring ohms and continuity
1. Always remove power from the machine otherwise you could blow your meter.
2. Always disconnect at least one side of any device you are checking. This eliminates the possibility of measuring an alternate/parallel circuit path.
3. When checking for closed contacts and continuity use the lowest scale (Usually 200 ohms). Then try higher scales. This scale is 0 to 200 ohms so if the device you are measuring is 300 ohms this scale would show an open circuit which it is not, you are just measuring outside the scale's dynamic range.
4. When you start always short the meter leads together. This will tell you that the meter is working and if there is any 0 offset.

There is a good STICKY at the start of this forum about it's use.