Q:

Not getting stronger at standing dumbell overhead press

It doesn't make any sense. I did 7 sets of 4 reps with 45 lb dumbbells each hand yesterday and today I tried to do a set with 50 lbs each hand for 2 reps and I couldn't do 2. I could only do 1 before getting stuck on the second (midway if that matters).
What am I doing wrong?

A:

Overhead work does tend to be more difficult to progress in than anything else.  First thing you'll need to understand is the concept of equivalent effort.  It's one way that many lifter's track progress, and decide if that 8 RM (Rep Max) was really a better effort than last month's 5 RM.
All of these are roughly equal effort to a 1 rep max:

95%, 2 reps
90%, 3 reps
87.5%, 4 reps
85%, 5 reps
80%, 8 reps
75%, 10 reps
70%, 12 reps

Now, if we do the math on the increase of weight we find:

45 / 50 = 0.9 = 90%

You've just jumped up with roughly a 10% increase.  When we account for the reps, we see that what we can best expect for a new 1 RM is:

45 / .875 = 51.4 lbs

(when figuring out an expected 1RM, divide by the percentage for equivalent effort).
Most of the time we do better with a 5% increase or lower.  As the weights get heavier the jumps are a lower percentage.  For example, jumping from 85 lbs to 90 lbs is just over a 5% increase and won't feel as incredibly difficult.
Dumbbells are a challenge because above 10-15 lbs the increases are all 5 lb jumps.
Recommended change
Until you can work with much heavier dumbbells you are probably going to have to push your reps more.
Go for at least 5 reps in a row if you want to be able to hit 50 lbs by 2.  Personally, I find pushing my reps out to 8 before going up in weight works out better for me.

50 / .95 = 52.6 lbs (target max)
45 / .85 = 52.9 lbs (should barely be able to get 2 at 50)
45 / .8 = 56.2 lbs (should easily make that 2 at 50)

Overshooting your effort with a lower weight gives more room to work with a heavier weight.  It also helps build momentum that will keep you more motivated.
Also Review Your Technique
Video yourself.  You'll find you can put more weight in your hands by getting better than you can just by working harder.

A:

Your first mistake was expecting progress after 1 day.
Your second mistake was doing the same exercise two days in a row.
After that, I can only guess about your form, but standing overhead press is usually something that doesn't come very naturally, so you will most likely have some mistakes there if you haven't recieved any guidance.

