I would have expected the bolus to have still been in the system after just over a month (max)?
I think you may have got it the wrong way around topdog. Bolusing a month before calving doesn't give the bolus enough time to work perhaps ? In any case from what you describe I'd say that the calf wasn't suffering from iodine deficiency. Probably Vit E or selenium ? Do you also have probs with retained afterbirths ?
I was told that iodine deficiency caused some infertility but it's main symptoms were cows being lazy at calving. We had many cases where we'd see a cow starting to calve with the water bag out etc. We'd then go back in say half an hour and find the cow lying down and chewing her cud without a care in the world. After getting them in to investigate we'd find the calf correctly presented and only a light pull was needed. Also the calves would sometimes be quite small and lethargic.
I would suspect that bouluses would be more effective if given in early to mid pregnancy, but I'm no expert. We bolus all our cows pre turn out, whether they're spring or autumn calvers and we rarely see any problems since we started using the boluses. The ones we use are Tracesure Cui.