** I can't see anything jammed up in there, so I'm wondering if something is out of alignment. **
No. Something is jamming it which apparently can't be seen with just a cursory look (ie. without dismantling it). It might need to be dismounted to get a more thorough examination.
** Should I call a mechanic or just scrap it and get a new one? **
If it is no longer under any warranty (see the following link) and there is not something just lodged in there, chances are it may not be economical to repair for your problem description in which case just replacing it might be the route to go.
LINK >
In-Sink-Erator Warranty Calculator
JMO
Dan O.
www.Appliance411.com
The Appliance Information Site
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