At 16dpo the average hcg level is around 200, however it can be lower and still go on to be a successful pg. Yes, I agree you'd hope to see 2-3wks on a digi, but they're not 100% reliable. But the digi result combined with your description of the lines on the other tests does make me suspect your levels are low. Best bet is the quantitative b/t from the Dr to check your levels. They should also do repeats to check that the numbers are rising appropriately and probably a scan as early as possible, given your history and risk factors.

FX all is well and you have a healthy little bean snuggled away in the right place.