The transmission is not building up enough pressure to shift. Check your fluid and fill if needed. If fluid is fine change the trans. filter and the pressure modulator. If that does not fix it have the tourqe converter flushed.
If that does not fix it, you may need to have the transmission overhauled or have the transmission replaced
Another thing, check your linkages!!. there could be a lose linkage on the transmission, or it could be seased.
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