Demand raised though there is sufficient amount in 26as...?

may be 26 AS is not the only criteria for raising demands....

Did you receive intimation u/s 143(1)? IT dept can make mistakes and if and when they do, the assessee is responsible to file rectification u/s 154 and let the AO know that 26AS has enough credit which will not give rise to any demand. 

The thing here is, though you have excess and enough credit in your 26As and the demand is rised means, the person who have deducted TDS in your A/c, might that the assessing officer of the concern TDS deductor have not updated the details to the department, so ask your deductor to check with their ward assessing officer, wether they have updated the details to the department for which the tds paid is credited to the relevant PAN. 

Even if the details at the concern ward assessing officer are updated and the situation is same as is, you take a copy of your 26AS of that particular period and enclosing it to a letter addressing to your ward assessing officer stating that the relevant demand of TDS was paid and that it is being reflected in the 26AS of that period in your PAN account and request that the demand may be dropped.

If possible take a copy of the Challan from your deductor of the TDS paid in the bank which have all the particulars of the bank like your PAN detail, BSR code of the bank, date of payment, amount paid, and the most important thing is the CIN. with these details you can check the same in the OLTAS and take a print of the same and enclose it to the letter and submit to your ward assessing officer.