Couple of questions:
is she a high producer?
is she losing weight/ looking a bit underweight?
how many days in milk is she?
If she is producing a lot of milk and is only relatively fresh she will be in negative energy balance which means she may have 'silent heats' or very rapid heats (some high producing cows may only get mounted once in their whole cycle-not enough to set off a kamar/estrus alert or produce a ruffled tail head). Also, cows less than about 80 DIM may abort their fetus early if they dont have enough energy to support a preganancy as well as high milk production and maintenance.
As far as CIDR programs are concerned, I have done them extensively on large dairies and often 10-20 % of cows bred dont show signs of heat but do still ovulate and get in calf.
A good preg tester should be able to palpate at 32-35 days after insemination and tell you if your cow is in calf. If she is not, they should also be able to tell you whether she has cystic ovaries or not (which can be responsible for cows who show excessive or constant estrus or no estrus at all depending on the type of cyst). Bear in mind that vets are only human so a second opinion is always valuable! Even ultrasound has a 4% error rate.
So if she seems happy and healthy but is still only reasonably fresh I wouldn't worry too much but if the lack of heats persists (after she has been preg checked) try another CIDR program but give a 5ml dose of GnRH instead of 1 ml as this will force ovulation in cystic ovaries.
Hope this helps!