Chain slipping, inner front ring only and can only use first 4 gears

Chainrings are 10 speed Shimano 105 double and rear cassette is Shimano 105 11-28. The chain is fairly new but the cassette has miles on it. If I'm in outer chainring I can select all 10 gears if I wanted to. If I'm in inner ring I can only select 4 biggest cogs closest to the spokes without slipping occurring. Chain isn't trying to jump to another cog, it's just like it's riding over the rear cogs from gears 5 to 10 if chain is on front inner ring.
Any thoughts on cause and what to do to resolve situation ?
Thanks
Any thoughts on cause and what to do to resolve situation ?
Thanks
0
Posts
How fairly new is the chain?
Only 10 speed cassette which doesn't need the spacer on a 10 speed freehub is Tiagra /HG500 which has it built in. So I'm guessing no.
I've known this for ages, but when I recently replaced a 10 sp 105 cassette with a Tiagra one I forgot all about it and left the spacer on the freehub. Miraculously I managed to tighten the lockring, but when I put the wheel in the bike the smallest sprocket was fouling the frame. And then the penny dropped...