Lost bass to rear 6x9s help

[SentraSounds]

I've had nice interior sound for almost 2 years and I don't push them hard but last week I noticed a major lack of bass from the rear. It has crystal clear highs no lows( not distorted just non exsistant) took it to the shop and they hooked up a working speaker got the same result. The said they hooked up a new head unit to the harness and same sound Please help in need of trouble shooting. Seemed like the shop I went to was just as baffled. ☘

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