I wasn't aware of EIS until recently. I have some small investments through crowdcube from 2019/2020 which I haven't claimed for in my tax return. Can I include them retrospectively on my current tax return?
Presumably you have received an EIS3 (or EIS5) form for the investments? (I doubt you can claim relief without one).
If you made an investment in shares issued during the year for which you have not yet received a form EIS3 or EIS5, you cannot claim relief for that investment. You must wait until you have received a form. If you receive the form after you’ve sent your tax return, complete the claim form inside the EIS3 or EIS5 and send it to us.
Presumably HMRC will look at your income in the tax year the EIS3 form was actually for, and apply the relief appropriately (and presumably generating a refund to you). As a general tax principle, you do not get to pick and choose the years in which you claim reliefs and to move things around for your convenience. However I see the link above does include a "Tax relief for a different year – your choices" section; the only option seems to be to claim the relief in an even earlier - not later - year though.