Fill a 10/11Ltr DI vessel 3/4 with resin, when TDS starts to rise, diss-connect it, let some water out ( to make a bit of room inside ) & give the vessel a good shake up/down - side to side for approx 30 seconds. this will get rid of any channels created by internal water flow & tds will go back to 000 once it has settled back down & a couple of Litres run through it. This has a better effect when virgin resin is used.
DA is right on the flow thing, blast water through a DI at mains pressure & the resin can't purify quick enough. However, a 10Ltr DI vessel filled 3/4 with resin will purify water easily with the small pumps we use for wfp, even at full flow rate.
tony