When multiple files exist in a filegroup, space is allocated from each file proportionally by the amount of free space in each file. Your new second file is only 1MB. If there is significantly more free space in the first file, then most new allocations will come from that one.

For example, if there's 500MB free in the first file, then new allocations from the new file only once for every 500 allocations from the first file.

In order to get data in the new file, grow it so the amount of free space in it can compete with the amount of free space in the first file.

Additionally, unless you measure data growth in kB/month or less, 1MB is a miserable starting file size. estimate what you may need in the next 6-12 months, grow the file for that now, and be done with it.