My spouse is German and has a tendency to get these two tenses reversed rather often when speaking English. This has given me reason to think a fair amount about these two tenses.
In English, the difference between the simple and the progressive is more of a dichotomy than in Spanish. In Spanish the lines are more blurred between these two tenses than in English.
Here's an example. I have a pet hamster. It is late evening as I write this and the little guy is running on his wheel compulsively right now. Note, I used the progressive. However, I could use the simple for the same observation, in Spanish. Here's a valid dialogue:
A: El hamster ¿ya se levantó?
B: Sí, ya corre, hasta más no poder.
In English, B would have to use the progressive: "Yes, he's running on his wheel, to beat the band." But in Spanish, it could be expressed either with the simple present (corre) or with the progressive (está corriendo).
Now, there are situations where you might want to use the progressive in order to make the continuity especially clear and explicit. For example:
C: ¿Por qué no llamas a la tía ahora para ver si quiere que vayamos por ella?
D: Estoy comiendo. La llamo pero un poco más tarde, o si prefieres, la llamas tú ahora.
You asked, "I'd like to know if there is ever an instance where you use the present progressive and can't use the simple present tense." The C-D dialogue is such an example.
Note, there is an even stronger way to convey continuousness -- with "estar en proceso":
E: ¿Por qué no envuelves el regalo para Bruno ahora?
F: Estoy en proceso ahora mismo, y casi termino.
Note what "en proceso" does to emphasize continuity:
G: Why don't you wrap Bruno's present now?
H: Yes, I'm doing that now, OR Yes, I'm in the middle of that now, and I'm almost done.