It can be, that the spacing between the chainrings is wrong.
What I mean is that say spacing between two adjacent chainrings (width 1.8mm) is different on 7/8-speed and 9-speed. I've just checked the spare parts and Miche advertises their 3mm spacers as suitable for 7/8- and 2.6mm spacers for 10-speeds Shimano. With this information I can conclude the following:
- the complete 7-speed cassette is 7 x 1.8 (chainring width) + 6 x 3 making it 30.6mm and this is what your indexed shifter expects.
- the 8-speed cassette it will still be 8 x 1.8 (chainring width) + 7 x 3 making it 35.4
- but for 9-speed cassette has the spacing reduced to 2.6 making 9 x 1.8 (the same chainring width) + 8 x 2.6 thus 37mm total
Now the fun part begins - you decided to use only 7 speeds of your 9 speed cassette. Assuming even width between the chainrings, 7 speeds out of your 9-speed cassette has the width of 7 x 1.8 (chainring width) + 6 x 2.6 making it 28.2mm which is 2.4 mm (or 133% of the chainring width, or one and one third of the chainring inward) misalignment.
If you align the derailleur on the middle gear it is still 1.4mm (or 78% of the chainring width) misalignment on both ends.
That is a lot and this misalignment and while adding some tension the chain slips as if you were trying to shift (I've experienced it last weekend on Eroica) but is cannot properly shift as the next chainring is also misaligned.
What did you do wrong? You've became a victim of spacer size change when going only one cog up.
The solution for you is to either:
- use 9-speed indexed shifter,
- use non-indexed shifter,
- come back to 7/8-speed cassette or
- rebuild your current 9 speed cassette with 8 speed spacers and most likely drop one chainring from it making it an 8-speed cassette effectively.