First, phosphate test kits are not very accurate for low concentrations, and unless you calibrate them, you can't rely on the test results. But, the plants will be using up the phosphates as soon as you add them, so having the concentration drop off is normal. Seachem's dosage recommendations seem to be for low light tanks, so if you have a high light tank you need to dose a lot more than they recommend. I think most people dose double their recommendation or even more.
Can you get something like Fleet Enema there - a phosphate based enema solution? If so, you can dose that for phosphates, but be sure it doesn't contain miineral oil or other active ingredients.