Many possibilities!!

- is there starch in the cactus? there is frequently some low amounts of residual starch co-extracted with pectins (e.g. with HNO3 at > 80°C, typical industry-like conditions), and it will not redissolve at room temp.

- I presume the pectins were EtOH (or PropOH) precipitated?  Not with calcium, copper or aliminum? If it were the case, residual ions would be a possibility.

- did you change the pH indeed? Solubility of pectins 5 is not the same; add to that the difference in temperature, it is very common that some pectins are not redissolved, you many need to increase the pH (but not too much), increase the temperature (idem, not too much), sonicate or just wait (I usually leave pectins to dissolve overnight at 4°C under stirring.

- The drying at 40°C is in itself tricky, if there was water left during the drying step (you must rinse with ethanol then acetone for best preservation of structure and solubility) you'll have encouraged formation of hydrophobic bonds, which will be hard to get rid of during redissolution in water.

- and last but very frequent, pectins just love to aggregate. And it is a dynamic phenomenon, i.e. maybbe they would have precipitated in the extract with a but of time.

Good luck!

Thanks for all these details.

i think that drying at 40°C/24 enhance the aggregation process, because lyophilized samples are more soluble.