Almost sounds like a bad site. I have had a problem before with a bad site to where the insulin is infusing between the fat layer and skin layer. It does not absorb well, but it may appear like it is bolusing well. BG may come down some, but usually the improvement you see is very minimal. The cannula may have been kinda positional where it wasnt completly blocked but over time it became that way. I would have changed the site out had I not seen significant improvement after correcting, especially if having corrected a couple of times with minimal improvement.
Also with Basal you wont necessarily get an alarm for no delivery, it is such a small amount infused that is why it doesnt trigger the alarms until you go to bolus. The larger amounts cause more pressure to back up and trigger the alarm. Has happened to me on occasion where first bolus seems to work, but over time and additional boluses the pressure level builds up and you get a no delivery alarm.