Mmmm! Have had a think about this one and can only think that your hCG levels must be too low for the blood test to pick up (as others have said).
On FF, if you have less than 2 charts for them to read, they suggest waiting until 18DPO to do a pregnancy test at the earliest, so maybe that's a good bench mark to go by.
As you are only 17DPO, your levels will most likely start to rise significantly over the next few days.
Hope so.
