The reason why thin upstrokes and thick downstrokes seem awkward is because this requires your hand to produce two different actions for the same letter. When the pen moves upwards, you must press lightly, yet when it moves downwards, you must allow the pen to be heavier on the paper. Next, the pressure must change once again before a curve or a loop or a connection is formed. This is an easy notion to understand but your hand must learn this action.
In many cases, thin upstrokes and thick downstrokes will be seen when the letters are formed with consistent pressure. You may see this when a student begins to form the letters because they want the letter to look well formed. However, if you write a letter upward with high pressure, the letter becomes large and cluttered. Similarly, in some cases, your letters may look pale, shaky, or broken on the downward strokes. Again, this is not a problem of ability but a problem of understanding when and how you should lighten your hand or when you should press your hand harder and longer.
It would help to begin with a single word that has only a few upstrokes and downstrokes such as minimum, hello, or name, rather than writing the whole letter in the word. Write this word again, once, at your own pace, and only concentrate on the light and dark of each letter. Ignore spacing, flourishes, or letter shape and simply look to notice the pressure that has been used. Then, circle the one upstroke that is too heavy and circle the one downstroke that is too light. This will give you a starting point on your letter, instead of writing the word again with all the changes you hope the letter will make.
The small pause between strokes is another thing that most beginning calligraphy learners have not yet noticed. It takes a small break between the heavy downstroke and the light upstroke, because this gives your hand a chance to let the pen down. This small break gives you time to relax before you press the nib into paper again. Also, when you lift the nib, or brush pen, your small break gives your hand time to lift it without smearing the line. However, the small break is not a stiff or a long break; it is only an almost breath-like action between two strokes.
It is also important to know which paper you are writing on. The paper you are writing on, and the kind of pen you are using, will determine how easy it is to create a light upstroke and a heavy downstroke. If the paper is rough, the pen may skip over it, and if the paper is very porous, the ink may run into the paper and the upstrokes may not really appear as thin. If you are writing with a brush pen, the pressure change should be easy; however, if you use a dip pen, the nib must have the right angle to the paper and the stroke must be slow enough so that the ink does not skip. Before you blame yourself, test out the pen by writing a few strokes to see if the pen skips.
Your hand tension also affects how you can create a light upstroke and a heavy downstroke. If your hand is stiff and the pen is not moving freely, then your upstrokes will appear very forced and your downstrokes will appear as though they are moving around on the paper, as if the movement is from your fingertips. This is why you should keep your hand in a firm position, but keep your hand and your wrists relaxed so that you have enough space to move the pen. If your hand feels stiff after a short time, stop writing the letters so that you can get your hand back to a more relaxed position, instead of continuing to move more.
It is important to recognize this change when the pressure change is either a little too long, a little too early, or not there at all, instead of recognizing the perfect change of light and dark in the letter. Write the word one more time, and only pay attention to where your hand should have moved from downstroke to upstroke. This is what helps you recognize your own progress, since this small practice is enough for your hand to know exactly where you need the downstrokes to appear light, and where your downstrokes should remain dark.