As Howard2nd pointed out, you’ve not specified which of several “ethernet” specifications you’ve based your efforts on. 802.3, 10BaseT or what?

Assuming 802.3…
1) Do you have the preamble in place? (7 bytes of 10101010)
2) Are you using basic or differential Manchester encoding?
3) Do you have the “Start of Frame” byte in place?
4) Are your frames at least 64 bytes long?
5) If not, have you padded them out to that minimum length?
6) What is the range of the packet being used to calculate the checksum?
7) Have you used an oscilloscope to verify presence or absence of electrical noise on the wire which could render the checksum invalid due to corruption of data bits?

Bob