There are a variety of things that could be causing this.
1. A tolerance issue (I'd be willing to bet you aren't new to methamphetamine considering the extreme ROA)
Try looking into ways at reducing this, a google search or search of the forums themselves should yield results
2. If you are binging (which for MA I would describe as multiple redoses over the course of 24+ hours) you may just be getting burnt out. Lack of sleep and proper nutrition can contribute to diminished effects and/or duration of said effects
3. Quality of the dope. Unless you washed all of your gear, and even then, you can never exactly assess the quality of the product. Sometimes even the most solid dealers will get a bad batch.
4. Your described ROA. Oral tends to last longer
Provide some more information and maybe I could help more.
Remember all of the risks associated with using the needle TweakerBirdie! Try a nice oral dose and see if it lasts any longer! I know the rush from IV can be pleasurable but I, along with many others, find this (oral dosing) the most sustainable and enjoyable experience.
First post over here at BL

Nice to be here. I'll be checking the thread for new replies best wishes to all. Stay safe TB