Wow, something actually really similar happens to me.
I started taking vyvanse 1 month and a week ago, in the beggining, i was taking 30mg and it lasted from 8 am to 5pm, but rapidly it started to work less and less. in a week, i was crashing at 1pm, so I told my doctor, and he changed me to 50mg. With 50mg i was crashing at about 3pm, but after 2 or 3 days i was crashing at 12am-1pm again (besides feeling much less concentrated during the time it was acting) so i took 200mg of DXM
as an NDMA antagonist, and the day after i took 50mg and it worked really well for about 10 hours. The following days, though, the effects slowly became more and more like before i took the DXM. You can call me irresponsible for this, i actually was, but i had exams, couldn't afford not to study, so i took DXM again a week after that and the effects once again lasted more and more effectively.
I will be seeing my doctor this week, and I will tell him about this and maybe suggest using a different NDMA antagonist dialy, such as magnesium or mamantine