James Hay has seen revenue decrease from £18.7m in the first half of 2012 to £18.2m in the first half of 2013.
In the first half 2013 results of its parent company IFG Group, the firm said the decrease was due to effects of the RDR.
Total revenue was the IFG Group increased by five per cent from £38m to £39m and poor results for James Hay were offset by revenue increases at its financial advice arm Saunderson House.
Saunderson House won 76 new clients and increased revenue by seven per cent from £12.6m to £13.4m.
The firm said: "Increases in revenue in Saunderson House and the Irish businesses more than offset a decline in revenue in James Hay Partnership, where the effects of attrition, new business lag and the once-off loss of income under RDR (Retail Distribution Review) had an adverse impact.
"The effects of attrition on the legacy James Hay book and the lag effect of new SIPP business are diminishing as the James Hay Partnership book has now achieved net book growth and continues to gain momentum."
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James Hay Sipp sales were 2,600 in the first half, exceeding the full-year sales of 2012 which were 2,469.
It now has 38,392 Sipps under administration and saw 2,600 new Sipps transfer into the firm and 1,550 transfer out.
James Hay's Modular ISipp, which was launched in January, continued to grow in popularity and the firm said it planned to launch further solutions for advisers and their clients.
Alastair Conway, chief executive of James Hay Partnership, said: "I am pleased to see sustained business momentum and the increasing investment in the business that we have been able to make.
"I am optimistic about the year ahead and expect a continued growth in new business sales as we continue to offer advisers a high quality service, unrivalled technical expertise and competitive and innovative products, all of which enable advisers and their clients to better plan for a secure retirement."