Appropriate regeneration of StrepTrap HP columns for FPLC

My question is related to protein purification using a ÄKTA FPLC.
We used StrepTrap HP Columns (1 ml column Volume (CV)) from GE Healthcare Life Sciences to purify a strep-tagged protein. In the first approach, this worked pretty fine.
We wanted to re-use the columns after regenerating them according to the official protocol. The included steps were all performed at a flow rate of 1 ml/min and with filtered solutions:

1. 3 CV distilled water
2. 3 CV 0.5 M NaOH
3. 3 CV distilled water
4. 5 CV 20 % EtOH
5. Storage of columns in 20 % EtOH at 4°C.
6. Equilibration of the columns with Binding Buffer just before next use.

By adhering to the protocol we encountered some problems with the next purification (it was the same protein, but a new sample of cell extract). The purification worked less effective and the amount of purified protein was very low.

We tried a slightly different protocol for regeneration, also at a flow rate of 1 ml/min and filtered solutions.

1. 5 CV distilled water
2. 5 CV 20 % EtOH
3. 3 CV 6 M Guanidinhydrochloride (sol.)
4. 5 CV 0.5 M NaOH
5. 3 CV distilled water
6. 5 CV 20 % EtOH
7. Storage in 20 % EtOH at 4°C
8. Equilibration with Binding Buffer before next use.

We chose the Guanidinhydrochloride to denaturate all remaining proteins from the column resin and support the regeneration by this.
But also with this protocol we encountered the same problems (low efficiency of purification).

I would be very interested if anyone of you have encountered the same problems and maybe have a solution/better protocol for this regeneration step.

Thank you for your suggests in advance! 🙂