number of samples and Amplitude accuracy,what's the solution to this problem I'm colleceting data samples from a device (temporal signal) in order to analyze it with FFT function in MATLAB R2015b,first i get N=4063 data points with (delta_t=0.000015s) so my frequency resolution is (delta_f=1/N*delta_t)=16.4Hz,this give me pretty good results with amplitude,but when i did another measurment with N=8192 data points (delta_t=0.000061)  to obtain better resolution,the amplitude is not correct,i don't know really how to deal with this issue.so please if any body can help me with this i will be grateful,thank you

ps:i'm new in signal processing

below my code in MATLAB

sig_2=asp_ach_de1;
Fs2 = 1/0.000061;      % Sampling frequency
T2 = 1/Fs2;            % Sampling period
L2 = 8192;             % Length of signal
t2 = (0:L2-1)*T2;      % Time vector
% FFT FUNCTION
Y2 = fft(sig_2(:,2))
Q2 = abs(Y2/L2);
Q1 = Q2(1:L2/2+1);
Q1(2:end-1) = 2*Q1(2:end-1);
f2 = Fs2*(0:(L2/2))/L2; %FREQUENCY
figure(3)
plot(f2,Q1)
I think that what is happening to u is that the frequency which you are trying to measure is in between 2 of your FFT bins which will cause the power to spread between two different indexes of your result FFT.

Another possible solution , only if with higher resolution you are getting a lower value  , is that since as I understand this is a real signal with noise you may be summing more noise , on the first attempt, for each frequency of your FFT and as a result raising all the values of your FFT. 

Without knowing the signal you are trying to process it is hard to know what is exactly the problem. also your FFT plots will help give a better answer.