Hello there DNA Artist,
there could be several reasons.
THe first is that you somehow have a longer DNA. Either by not cutting fully or by PCR mistakes (it is longer than 70bp).
The other reason is that you have degradation, meaning that you get a smear instead of a clear band.
To get a clean RNA product, I would make sure you cut the DNA 100% to completion to have runoff transcripts and also use a sephadex column to get rid of nucleotides and small unfinished (but partly transcribed) RNAs.
I used RNAse free-Sephadex G-25 for less than 200bp RNAs (Roche carries them).
please let me know how you are doing.