Translation:She was doing experiments but she did not understand the results!
They are all the same tense in Romanian, but the translations to English change and hence it is confusing in certain phrases. "She was doing experiments but she wasn't understanding the results" is the literal translation, but "wasn't understanding" does not sound good in English.
I don't agree, "she wasn't understanding the results" is the correct tense to match the rest of the sentence and sounds perfectly grammatical and fine to me (native English speaker). E.g. "She was doing experiments but wasn't understanding the results so she hired a tutor".