After about 30 minutes of searching, I've come up empty-handed in terms of finding a suitable scientific reference in answer to your question.
However, according to this resource, in tandem learning:
Both partners are responsible for their own learning - in other words, they both decide on the goals and methods for their part of the exchange. Two learners will rarely have exactly the same objectives and approach, so it is important that they discuss their goals and needs with each other, and then set about achieving them in their share of the tandem time.
Tandem partners are both experts for their respective language and culture. Together they may choose to read aloud, discuss particular topics that interest them, correct mistakes, suggest improvements, practise conversations or undertake any number of other activities.
One of the most important aspects for successful tandem learning is that the partners meet as often as possible. This helps learners to become familiar with the foreign language more quickly, increases learning frequency, and makes sure that they get plenty of practice.
To make tandem sessions as interesting as possible for both partners, they should deal with topics that are of equal interest to both learners. In other words, it is important that tandem partners find shared interests to discuss. While goals may vary, tandem language learning itself should first and foremost be an enjoyable experience!
This passage provides a basic overview of what is needed for a successful tandem learning session. Thus, by working backwards, we may be able to find out why you had an unsatisfactory experience. Here are some possible reasons I can decipher from the quote above:
- Your partner may not have been upfront about their goals and objectives, and may not have asked about your own goals, so by generalizing, your partner did not cater to your own learning style/approach.
- Your partner may not be extremely knowledgeable in their language or culture, even if they are a native speaker.
- You may not have had consistent and regular meetings. Often consistency is the key to learning a new language.
- You should be discussing topics that are interesting to both of you to prevent one of you falling asleep in the middle of the conversation.
Keep in mind that although, I couldn't find many opinions on this, tandem sessions are essentially just immersion techniques. It may be possible to learn a language through only immersion, but if you want to expedite that process and also retain a better understanding of the language, studying and practicing regularly will go a long way.