Hard shifting 1st to 2nd

New problem with van - when shifting from 1st to 2nd, there is a pause before 2nd kicks in, then it grabs when it connects. 2nd to 3rd OK. Then when downshifting from 2nd to 1st, same pause and clunk when 1st connects.
Fluid is fine - don't know anything else to check before heading to the mechanic.
Any ideas what could be causing the problem?
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