I cannot get my chlorine level to go up.  They are normally around 4-5 ppm and last week they dropped to 0.  I dropped in a pound of Dichlor last night and chlorine shot up, but was gone about 3 hours later. The pool is small at only 6000 gallons.  The T15 SWCG is now at 50%, clean and voltage/amps are 25v at 7.3 amps.  Any idea what I'm missing?  It normally ran in the 20-25% range with no issue and is only 4 months old.  Below tests are after running at 100% for 8 hours last night.  Water is very clear, no visible signs of algae.

TC: 0

FC: 0

pH: 7.4

TA: 80

CA: 325

Stabilizer: 50

TDS: 5000

Salt: 3500 (just added over weekend as it was down to 2800)

Pool store suggested superchlorinating at 100% for 24 hours and re-testing.