I have noticed this lately... I have had to take 100mg instead of the 50mg I usually take quite often here lately. This isn't all the time as the 50mg will take care of it sometimes...
I've been taking Imitrex (oral) for almost 4 years and in the last year it stopped working like it used to. But it may also have to do with how often you take it. My migraines got a lot worse in the last couple years and I was taking Imitrex 3 times a week... and most likely building up the tolerance quickly.
It may all depend on how often you have migraines (how often you need to take the Imitrex) and how much your dosage is?
I've only had luck with taking Imitrex orally... never tried the injection and didn't have any luck when trying out the nasal spray.
Hope I could help!