Hi,

If you think you have too much debris pellet and suspect partial lysis you can try change your lysis method, I'd rather move to rotor stator while paying attention to avoid foaming during homogeneization. This can be done either on the powder obtained with mortar and pestle or directly on the tissue sample.

I've also often used a phenol/kit hybrid method with tissue homogeneization in acid phenol that foams less than RLT, and aqueous phase purification on column on various tissues like kidney, liver, heart, skeletal muscle.

I guess increasing time in RLT buffer would not change much as its role is, as far as i know, to provide a strong denaturating environment with the chaotropic properties of guaninide thiocyanate to avoid degradation by the RNAses of the sample during homogeneization before getting rid through the column.

Hope this helps,

best,

Daniel