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Do not use any amplifier. Signals are already calculated (and assumed) to be moderately strong. Inappropriate amplification often opens up the system to new problems.

Changes in reception over time are always due to changes in either the system or it's local environment over time.

Corrosion or water ingress can cause degraded reception. Not likely for an attic install, though.

Most common problem, though, is the growth of trees in or near the signal path over time. Due to the "cliff effect" inherent to digital TV, reception will appear normal right up to the point where it isn't normal anymore. Usually, degraded reception starts with only one or two channels whose frequencies just happen to be the ones most drastically affected first. You'll need to look at your signal path with a critical eye for those kinds of details. Moving the antenna usually changes the results.

Other changes in the immediately local environment (construction, local noise sources, etc) can alter the signal path or environment and also cause degraded reception.