Using SQL Exists but having issues - sql-server

Question

I have a stored procedure which inserts records from a lab, and for months it has been inserting records and working. But in the last month something has changed and my record check stopped working.
The SampleNames exist and I can do a query to locate the sample name. What might be the issue?
I can get it to sort of work if I replace the If Exists with this:
IF EXISTS (SELECT [SampleName] FROM [dbo].[HPLC_Sample]
WHERE [SampleName] LIKE '%' + UPPER(#SampleName) + '%')
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[sp_HPLC_Params]
#SampleName nvarchar(32),
#DataFile nvarchar(max),
#AcqInstrument nvarchar(50),
#AnalysisDate datetime,
#ResultsCreated datetime,
#ResCreatedBy nvarchar(50),
#AcqMethod nvarchar(50),
#InjDate datetime,
#AcqOp datetime,
#SeqLine int,
#Location int,
#Inj int,
#InjVol int,
#ActualInj int,
#SeqFile nvarchar(50),
#StartPress float,
#StopPress float,
#StartFlow float,
#StopFlow float,
#SortBy nvarchar(50),
#CalbDTCreate datetime,
#CalbDTMod datetime,
#PeakID nchar(10),
#Muliplier int,
#Dilution int,
#UnCalPeaks nchar(10),
#NumSignals int,
#NumErrors int
AS
BEGIN
DECLARE #SampleID AS smallint
IF EXISTS (SELECT [SampleName] FROM [dbo].[HPLC_Sample]
WHERE [SampleName] = #SampleName)
BEGIN
SELECT #SampleID = [SampleID]
FROM [dbo].[HPLC_Sample]
WHERE [SampleName] = #SampleName
END
ELSE
BEGIN
INSERT INTO HPLC_Sample([SampleName])
VALUES (UPPER(#SampleName))
SELECT #SampleID = [SampleID]
FROM [dbo].[HPLC_Sample]
WHERE [SampleName] = #SampleName
END
END
BEGIN
SET NOCOUNT ON;
BEGIN
INSERT INTO HPLC_Params([SampleID], [Data File], [Acq Instrument], [Analysis Date], [Results Created],
[Results Created By], [Acq Method], [Injection Date], [Acq Operator],
[Seq Line], [Location], [Inj], [Inj Volume],
[Actual Inj Volume], [Sequence File], [Start Pressure], [Stop Pressure],
[Start Flow], [Stop Flow], [Sorted By], [Calib Data Created],
[Calib Data Modified], [Peak ID], [Multiplier],[Dilution],
[Uncalibrated Peaks], [Number of Signals], [Number of Errors])
VALUES (#SampleID, #DataFile, #AcqInstrument, #AnalysisDate, #ResultsCreated,
#ResCreatedBy, #AcqMethod, #InjDate, #AcqOp,
#SeqLine, #Location, #Inj, #InjVol,
#ActualInj, #SeqFile, #StartPress, #StopPress,
#StartFlow, #StopFlow, #SortBy, #CalbDTCreate,
#CalbDTMod, #PeakID, #Muliplier, #Dilution,
#UnCalPeaks, #NumSignals, #NumErrors)
END
END