Did you check for inclusion bodies?

Is your conclusion based on SDS-PAGE analysis of cells, cleared lysate, ...? If your protein is a membrane protein, it may be more difficult to get high yields and to see it using SDS-PAGE analysis of a small amount of cells (as is often done).

Perhaps check whether there is a growth defects after protein induction (compared to empty-vector control) in case your protein has a "toxic" effect on E. coli.

I agree with Omar, knowing if the protein forms inclusion bodies is important.

I agree the comments given above. In addition, I need to raise some points for you to think about them.

Did you observe the growth of cells? If cells are growing without expression of your target proteins, you can optimize expression conditions.

You need to consider all the critical factors affecting protein expression. The important parameters that should be considered include:

1. Expression vectors

2. Expression host cell strains

3. Type of culture media, e.g. LB vs TB

4. Concentration of IPTG

5. Post induction temperature

6. Induction OD

7. Post induction time

8. Presence or absence of media additives

9. Co-expression of chaperones

10. Use of solubility enhancing fusion tag proteins etc...

You can use a factorial design to test the effect of some of the critical parameters at different levels.

Go for codon optimization in case you opt for cloning again. Check for inclusion bodies, standardize your IPTG concentration and cell lysis conditions. Try to maintain the ambient temperature while handling proteins. Higher temperatures tend to overexpress the protein in IBs.

Good luck!