1. Check if both SO-DIMMs are firmly inserted into memory slot, if not fix it and check if now the 16GB is recognized.
2. If you didn't see any thing wrong in the memory stick installation, remove one memory stick and check if the remaining memory is recognized.
3. Now, move the remaining memory stick to the empty memory slot and check if in this position this memory stick is recognized.
4. Repeat para. 2 and 3 for the other memory stick.