First of all a good answer would depend on knowing more. Among other things is how quickly it goes flat
Assuming a fairly slow leak, not from a glass puncture, it's a rim leak, either at the valve, spoke hole, or under the tire bead.
Since sealant tends to circulate unde the tread, rather than towards the rim, try pumping the tire and slowly turning the wheel while near horizontal. This will circulate sealant, hopefully helping it find and seal at the rim/bead area.
If no luck, then unmount the tire and examine the rim looking for any dings or burrs at the tire seat, and of course checking the tape, especially at the valve.
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