When Ethica captures data from the participant's device, it encrypts the data and stores it on the phone as long as necessary. The data will be removed only when it's uploaded to the servers. If a given participant does not have Internet connection for a long period (2 weeks, 2 months, or even longer), the data accumulates on her phone, and as soon as a connection is available, they are uploaded.
There are two pitfalls to be aware of here: first, when a large amount of data is stored on the device and not uploaded yet, the chances of losing all the data because of phone hardware issues increases. The person might lose the phone or might break it, and in that case all buffered data are lost.
Also, if large amount of data are buffered on the device, the device might run out of space to record new data. This is particularly a problem with low-end older phones which have smaller memory.
So while the app does it's best not to lose any data, it's always good to make sure the data is received from all participants regularly.